The Evolution of Irregular Warfare

Commentary by Daniel Byman, Seth G. Jones, and Sofia Triana

Published September 16, 2025

This commentary is part of a report from the CSIS Defense and Security Department entitled War and the Modern Battlefield: Insights from Ukraine and the Middle East.

The wars in Ukraine and the Middle East offer many lessons for better understanding, conducting, and countering irregular warfare.¹ On October 7, 2023, the Hamas attack on Israel combined attacks on Israeli military bases near Gaza, border security infrastructure, and military communications equipment with atrocities against Israeli civilians and the taking of civilian hostages. Russia, for its part, accompanied its February 2022 invasion of Ukraine with cyberattacks, attempts to kill President Volodymyr Zelensky, and a deepfake in March 2022 to try to encourage Ukraine's surrender. Ukraine has used guerrilla attacks, sabotage, and leadership assassinations to fight Moscow. Some combination of these and other forms of irregular warfare is likely in future conflicts.

U.S. and allied planning, posture, and doctrine must prepare for irregular warfare, incorporating the impact of civilians and recognizing the vital roles of special operations forces and intelligence services in conflict.

Drawing on the lessons from the Ukraine and Middle East wars, this chapter makes the following arguments about irregular warfare:

1. Irregular warfare often occurs as a prelude to, or side-by-side with, regular warfare and can inflict many casualties, undermine resilience, and raise the price of occupation.
2. Civilians are often at the heart of irregular warfare-as shields, as victims, and as targets of coercion-and governments must consider this when confronting opponents who use irregular warfare and in their own irregular warfare operations.
3. Intelligence is critical to counter irregular warfare, as Israel's successes show, and in general an effective response can limit the coercive power of irregular warfare.

U.S. and allied planning, posture, and doctrine must prepare for irregular warfare, incorporating the impact of civilians and recognizing the vital roles of special operations forces (SOF) and intelligence services in conflict. This, in turn, will require adaptation, including recognizing differences between irregular warfare involving great powers (as compared with past U.S. efforts against weaker insurgencies and terrorist groups) and ensuring that private sector technology and expertise are incorporated into U.S. efforts.

This chapter has three sections. The first section notes several lessons from the Ukraine and Middle East wars; the second examines Israeli and Ukrainian successes regarding irregular warfare; and the third discusses the implications of these lessons for the future of warfare.

Lessons from Ukraine and the Middle East

The experiences of Ukraine and the Middle East offer many lessons on how to think about irregular warfare now and in the future. First, irregular warfare often occurs side by side with conventional warfare, and it is necessary to prepare for the two happening simultaneously as well as in isolation. Second, the death toll and other costs of irregular warfare can be high, especially for enduring conflicts. Third, hostage taking, terrorism, assassination, and other means of conducting and fighting irregular warfare are often part of broader efforts to coerce and deter opponents.

Irregular and Conventional Warfare in Tandem

In both the Ukraine and Middle East wars, irregular warfare has occurred simultaneously with regular warfare. In parts of Ukraine occupied by Russia, Ukrainian partisans, directed by Ukrainian special operations forces, used guerrilla attacks to kill Russian forces, disrupt lines of supply and communication, and sabotage Russian weapons systems. These efforts disrupted the flow of military supplies and forced the Kremlin to divert resources from the front lines to the repair and defense of its rail infrastructure instead, placing additional strain on an already struggling railroad network. Ukrainians have also used nonviolent resistance, such as wearing yellow ribbons in solidarity and distributing information to counter Russian propaganda.² Overall, Ukraine's efforts have hindered the movement of Russian troops and created supply bottlenecks.³ More importantly, they have also prevented Russia from successfully incorporating captured Ukrainian territory into Russia. The cost for Ukrainian civilians is high: The United Nations reports over 12,000 civilians have died so far, including many in territory occupied by Russia.⁴

Ukraine has also targeted Russian warships in the Baltic Sea as well as railway networks, blowing up the Sveromuysky tunnel in eastern Russia and damaging a critical railway bridge near the city of Kinel. In the case of the Sveromuysky tunnel attack, Ukraine's Security Service reportedly sabotaged a train's fuel tank, causing it to catch fire as it moved through the tunnel. Other trains scheduled to go through the tunnel were then rerouted to a bridge where they were damaged as explosive devices planted along the alternate route promptly detonated.⁵

In the Middle East, some groups, like the Lebanese Hezbollah, have integrated irregular approaches to warfare into their order of battle and military doctrine. Hezbollah has long fought Israel with rocket and missile strikes, guerrilla warfare, and terrorist attacks, and it has also trained groups-like Hamas-that have a similar set of capabilities, if less powerful. Israel, which has mostly fought a conventional war against its opponents, nonetheless has mixed a conventional invasion of Gaza with leadership strikes on Hamas throughout the Middle East and Hezbollah in Lebanon.

The High Cost of Irregular Warfare

Irregular warfare is often considered a weapon of the weak, yet it can still inflict considerable costs on a strong opponent. Hamas was undeterred by Israel's military superiority and killed around 1,200 Israelis-mostly civilians-on October 7, inflicting an extremely high number of casualties on a small and casualty-sensitive country. Over 400 more Israeli soldiers have died in subsequent combat in Gaza, where Hamas has used hit-and-run attacks, improvised explosive devices (IEDs), and other indirect means to inflict casualties while avoiding a direct confrontation with the better-armed and better-trained Israel Defense Forces (IDF).⁶ The ensuing conflagration has­­ similarly led to the deaths of some 60,000 Palestinians, further illustrating the high costs of irregular warfare. In addition to the death toll in the Gaza war, the Hamas attacks pushed Israel into war not only in Gaza but also in Lebanon and Yemen.

Such irregular warfare measures have raised the price of occupation. Fighting insurgents, especially in densely populated areas like Gaza, requires a grinding counterinsurgency with high force levels. For the Gaza war and other Middle East conflicts, Israel mobilized some 360,000 reservists.⁷ As of August 2025, Israel has conducted a 22-month war to suppress Hamas, yet the group remains the strongest organization in Gaza. Similarly, Russia has not fully pacified the territory it occupies.⁸

Irregular Warfare as a Tool of Coercion and Deterrence

The threat of irregular warfare can also be used in attempts to coerce and deter. Iran, for example, relies heavily on Hezbollah and other proxy groups to impose costs on Israel, the United States, and its Arab enemies. The threat of Hezbollah rocket and terrorist attacks was in part meant to deter Israeli operations against Iran itself. In addition, Iran-backed groups like the Houthis attacked Red Sea shipping to coerce Israel into ending its war in Gaza. Russia has also engaged in a comprehensive campaign of sabotage in Europe to punish countries that supported Ukraine and limit future support. Moscow's increasingly brazen attacks have included jamming GPS systems to disrupt civil aviation, causing deliberate damage to undersea gas pipelines and telecommunications cables, sabotaging water utilities in Poland and France, and conducting arson attacks in the United Kingdom, Czech Republic, Germany, Lithuania, and Latvia.⁹ Russia has also targeted facilities with more direct links to the war in Ukraine, including a BAE Systems munitions factory in Wales and a U.S. military base in Bavaria.¹⁰

Hostage taking has proved an important part of both the Gaza and Ukraine wars. In Gaza, Hamas and other Palestinian groups initially took 251 hostages-including children, the elderly, and other noncombatants as well as many non-Israelis-and, as of August 2025, around 50 are still in captivity, although more than half of these are presumed dead. The presence of hostages has complicated Israeli targeting and offered a form of protection for Hamas leaders. In occupied Ukraine, Russia has engaged in forced deportations of almost 20,000 children to Russia, placing them with Russian families and refusing to return them to their Ukrainian relatives.¹¹

In part because irregular forces hide among civilians, countering irregular warfare can involve considerable death and suffering in the civilian population. Hamas fighters have blended in with Gazan civilians and hidden arms and fighters in civilian infrastructure, such as hospitals and schools. Israel's response has been devastating for ordinary Gazans, with over 60,000 Gazans killed in total as of August 2025, most of them civilians, as well as most of Gaza's infrastructure destroyed. Operations that involve numerous civilian casualties place an additional burden on democracies, which are more likely to receive criticism when their military operations involve civilian deaths.

Israel has devastated Hamas and Hezbollah through assassinations, and both Russia and Ukraine have used assassinations as well. Although Ukrainian authorities rarely claim responsibility for their covert actions, they have carried out high-profile assassinations in occupied Ukrainian territories as well as on Russian soil. Among the individuals successfully targeted by Kyiv are Vladlen Tatarsky, a Russian military blogger; Igor Kirillov, the chief of Russia's radioactive, chemical, and biological defense forces; and Illya Kyva, a pro-Russia former Ukrainian member of parliament who fled to Russia during the war. Ukraine has also targeted leaders in occupied Ukraine who collaborated with Russia.¹² Moscow, for its part, has also undertaken a broad campaign of targeted assassinations in Ukraine and across Europe, poisoning the wife of Ukraine's military intelligence chief, killing a senior Ukrainian covert action leader, plotting to assassinate the chief executive of German arms maker Rheinmetall, and gunning down a Russian military defector in Spain.¹³

Countering Irregular Warfare

Although irregular warfare is difficult to combat, both Israel and Ukraine have scored many victories. The Lebanese Hezbollah, one of the world's premier guerrilla organizations and one that fought Israel to a standstill in their last all-out clash in 2006, largely failed in its use of irregular warfare against Israel and ended up taking tremendous losses. Israeli intelligence deeply penetrated Hezbollah, sabotaging its pagers and walkie-talkies and gaining precise information on the locations of Hezbollah leaders. With this intelligence, Israel was able to decimate Hezbollah's senior leadership, including killing the group's longtime secretary general, Hassan Nasrallah, and inflicting significant losses on its rank and file. Israel also successfully targeted much of the group's rocket and missile arsenal. This stockpile, estimated to contain between 120,000 and 200,000 projectiles, was reduced by half due to Israeli airstrikes.¹⁴ Hezbollah was forced to sue for peace, ending its attacks on Israel and agreeing to withdraw its forces from the Lebanon-Israel border, with Israel making few concessions.

Iran's ties to Hezbollah, militant groups in Iraq, and the Houthis did not deter the United States or Israel from acting against it militarily. Israel in particular targeted Iranian military leaders in Syria and Lebanon and the leader of Hamas when he was in Iran. Tehran did try to restore its credibility with drone and missile attacks on Israel, but this too was a failure, with Israel-helped by the United States, Jordan, and other countries-tracking and downing most of the attacking force. When Israel and Iran fought a bigger battle in June 2025, Hezbollah avoided joining the fray.

Hamas's seizure of Israeli hostages has likewise not proven an effective deterrent. Despite the presence of over 200 hostages, Israel launched an all-out assault on Gaza, and in its operations has conducted highly destructive attacks that have threatened the hostages as well as their Hamas kidnappers. Israeli ground forces have also accidentally killed hostages.¹⁵

Finally, Israel and the United States have prevented Iran from escalating irregular warfare into conventional success; indeed, Tehran's efforts to do so have led to embarrassing failures. After the killing of Iranian, Hezbollah, and Hamas leaders, Iran twice launched large salvos of rocket, missile, and drone attacks on Israel, and Israel responded with limited but precise attacks-the first time Iran and Israel have directly attacked each other's territory. Effective intelligence and air defense, however, prevented Iran's salvos from causing significant casualties in Israel, displaying Tehran's conventional military weakness in a highly public way. In part due to threats from Israel and the United States, Iran also hesitated to escalate further and counseled some of its proxies, such as those in Iraq, to limit attacks on U.S. bases.¹⁶ When Iran and Israel (joined by the United States) entered into the larger conflict in June 2025, Israel was quick to gain air supremacy and, in a short but effective air campaign, set back Iran's nuclear program and killed many Iranian leaders, with only a small number of casualties on the Israeli side.

Implications for the Future of Irregular Warfare

During the Cold War, the most frequent type of competition between the Soviet Union and the United States was irregular warfare, as the two sides fought proxy wars in Africa, Asia, Europe, and Latin America. The same may be true in the coming years as China expands its global presence. Although a Chinese invasion of Taiwan is possible, more likely are cyberattacks, disinformation, sabotage, and military threats to coerce Taipei and undermine morale.

In addition, the staggering cost of the Ukraine war in both money and lives suggests that an exhausted but predatory Russia may in the future prefer to use irregular war instead of conventional attacks to expand its influence. Russia's Main Directorate (GRU), Foreign Intelligence Service, semiprivate military companies, and other state and nonstate organizations are likely to continue assassinations, sabotage operations, offensive cyber campaigns, disinformation operations, intelligence collection, and other clandestine activities. The GRU's Service for Special Activities is likely to be particularly active, including Unit 29155 (also known as the 161 Center or, more formally, the 161 Intelligence Specialists Training Center), Unit 54654, and the GRU's headquarters and planning department.¹⁷ Russia will also likely continue to wage a disinformation campaign against the United States, conduct offensive cyber campaigns against U.S. and Western government agencies and companies, and engage in a range of other activities such as assassinations and sabotage.

Iran, for its part, emphasizes irregular warfare given the weakness of its conventional forces. It will continue to pose an irregular warfare threat to the United States and its allies and partners across the Middle East using a range of partner forces such as the Houthis in Yemen, Hezbollah in Lebanon, Hamas and other groups in the Palestinian territories, and the Popular Mobilization Forces in Iraq. In addition, Iranian government entities such as the Islamic Revolutionary Guard Corps, as well as their nonstate partners, will likely improve their offensive cyber capabilities and their ability to conduct attacks against the United States and its allies and partners at home and abroad. Although Iran and its proxies' setbacks in 2024 and 2025 will make Iran more hesitant to take on Israel, Tehran has little choice but to fall back on irregular warfare, as its conventional forces are poorly armed.

In addition to excelling at high-end conflict, the United States and its allies must be prepared for irregular operations with attacks on civilians and the use of civilians as shields, ensure there are civil affairs officers who can repair civilian infrastructure, create partnerships with private sector companies with cyber and other expertise, and develop other capabilities to better counter irregular warfare.

Even as the United States emphasizes great power competition, it must not lose the knowledge gained after its interventions in Afghanistan, Iraq, and other parts of the world in the post-9/11 era-as happened when the U.S. military deliberately tried to put the Vietnam War behind it and, at high cost, had to relearn how to fight insurgencies. In addition, unlike in the Vietnam era, insurgents and other irregular forces may have great power support, including better weapons, funding, and intelligence. There is also a risk of escalation that must be managed when irregular forces have a great power sponsor.

Fighting irregular opponents often risks large numbers of civilian deaths. In some theaters there will be media and international scrutiny of the impact of military operations on the civilian population. Countries fighting in these regions will require a media and public relations strategy to go along with their operations, all while targeting procedures that seek to minimize harm to civilians. In addition, countries must be prepared for disinformation about their operations, specifically regarding their harmful impact on civilians.

Assassination and sabotage are likely to remain part of irregular warfare, both on offense and defense. If the Russia-Ukraine conflict is a guide, some of these assassinations are likely to occur far from the front lines, requiring new security protocols in more remote bases and even in faraway homelands. Sabotage of U.S. bases and supply lines, as well as those of allies, is also highly likely.

SOF will play a particularly critical role in combatting irregular warfare in the future. SOF need to adapt given the many differences between fighting against forces of or supported by a great power versus fighting terrorists. Russian forces, for example, have persistent surveillance and airpower that will make clandestine operations against them far harder for U.S. forces compared with U.S. efforts fighting terrorist groups. It will also be important to develop programs to raise forces to gather intelligence and fight behind enemy lines. Hostage rescue may also be required, even as military operations occur in close proximity.

Success in irregular warfare requires superb intelligence. Targeting adversary leadership (and protecting one's own) necessitates detailed information on leadership movements and communications. Striking irregular forces while limiting harm to civilians also requires excellent knowledge about the locations of fighters and the presence of nearby civilians. Sabotage, such as what Russia is currently conducting in Europe, needs to be disrupted, attributed, and called out to rally unified allied support. In addition, some intelligence may need to be released to counter claims that, for instance, the United States has targeted civilian infrastructure without military purpose.

Authoritarian states are also vulnerable to irregular warfare, of course, including information warfare. By leveraging commercial technologies, the United States and its partners should target the domestic populations of China, Russia, Iran, and other countries through covert, clandestine, and overt means, where appropriate. The commercial sector can be helpful in developing and utilizing AI, large language models, and software that directs information to specific audiences that Beijing, Moscow, Tehran, and other regimes are attempting to control. Offensive information operations could focus on a range of issues, including domestic grievances and societal divisions, human rights abuses, economic problems, and corruption.

Military operations and intelligence units will likely need to develop greater capabilities to compete in the information space, including for such activities as covert influence and counter-value operations (targeting an adversary's civilian population). In cooperation with the commercial sector, AI and large language models have significant potential for irregular warfare applications. AI translation and message crafting can provide government officials with the ability to rapidly communicate in any language with anyone in the world. Advances in natural language processing will accelerate intelligence work, helping analysts sort through reams of text and drawing connections a human brain might not.

The military and intelligence communities need to fundamentally change the way they work with the commercial sector to compete more effectively in irregular warfare-both on offense and defense. Commercial innovation and commercial production capacity provide a major advantage for the United States and its allies and partners in irregular warfare, including for intelligence and military-related activities. But the United States has not adequately leveraged this advantage because of risk aversion, slow and burdensome contracting and acquisitions regulations, and a failure to adequately understand viable options in the commercial sector. There is a significant need to rethink the framework of government collaboration with this sector and to treat commercial entities as partners serving a common goal.

There is also a growing need to improve next-generation intelligence platforms, systems, and software that can quickly collect and analyze vast amounts of information on adversary activities for irregular warfare. Adversaries will likely attempt to hide their actions in a variety of terrains, including jungles, mountains, dense forests, subsurface locations, and tightly packed megacities. They will also attempt to use denial and deception tactics and techniques.¹⁸

Finally, an important goal is to limit the escalation of irregular warfare into conventional conflict. This can occur when major powers feel the need to respond to attacks on their proxies or when proxy attacks compel their targets to respond against the ultimate source. Israel and the United States achieved this with Iran in 2024, where Tehran's fear of U.S. and Israeli escalation led Iran to try to calibrate its initial attacks to avoid escalation and to avoid additional attacks after its failed drone and missile salvos.¹⁹

Conclusion

The wars in Ukraine and the Middle East demonstrate that irregular warfare is not a relic of the past, but a defining feature of contemporary conflict-one that democratic states must be institutionally and operationally prepared to confront. Civilians are often the primary victims, caught between actors that deliberately use population centers for tactical advantage and militaries that must operate under intense legal and normative scrutiny. Indeed, in dense urban environments like Gaza City, civilians are often used as shields, and in Ukraine, noncombatants are the principal victims of coercive tactics intended to undermine resilience and morale. The persistent threat of assassination, sabotage, and hostage taking-often executed through or with support from intelligence and SOF-will remain a central feature of irregular campaigns. As adversaries grow more adept in their use of irregular means, democracies must invest not only in better intelligence, cyber defense, and targeting capabilities, but also in public communication strategies to counter disinformation and preserve legitimacy.

The wars in Ukraine and the Middle East demonstrate that irregular warfare is not a relic of the past, but a defining feature of contemporary conflict-one that democratic states must be institutionally and operationally prepared to confront.

Still, the wars in Ukraine and the Middle East have also demonstrated that well-coordinated efforts can reduce the impact of irregular warfare. Ukraine has disrupted numerous plots to assassinate President Volodymyr Zelensky. For Israel, timely and effective intelligence allowed it to decimate Hezbollah's ranks and quickly neutralize massive Iranian drone and rocket attacks.

Strategic adaptation is essential. The United States and its allies must preserve hard-won knowledge from post-9/11 counterinsurgency operations while recognizing that great power-backed irregular warfare poses far more sophisticated challenges than ever before. This includes preparing SOF for operations against technologically capable adversaries, building rapid and resilient intelligence-sharing platforms, and rethinking how the government works with commercial innovators to harness advances in AI and data analytics for irregular conflict. Future military operations will require increased readiness for irregular methods such as assassinations and sabotage, excellent intelligence, better cooperation with the private sector, and preparation for irregular warfare in an environment of great power competition. Future success will also depend on mitigating escalation risks-particularly when attacks by proxies or in the gray zone threaten to pull major powers into direct confrontation. The lessons from Ukraine and Israel point to a critical imperative: Irregular warfare is not only a tactical reality but a strategic domain in its own right, and ignoring it would be a grave miscalculation in an era of persistent geopolitical competition.

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Daniel Byman is the director of the Warfare, Irregular Threats, and Terrorism Program at the Center for Strategic and International Studies in Washington, D.C. Seth G. Jones is president of the Defense and Security Department at CSIS. Sofia Triana is a program coordinator and research assistant with the CSIS Warfare, Irregular Threats, and Terrorism Program.

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