What Does Lethality Really Mean in Modern War

Critical Questions by Benjamin Jensen

Published October 2, 2025

Secretary of War Pete Hegseth, a combat veteran, is asking the right question: What increases lethality in a modern military? Pundits and analysts can disagree about the answers and the manner in which Secretary Hegseth presented his case, delivered to hundreds of general officers on September 30, 2025, but the validity of the question remains.

This article explores how the U.S. military and broader strategic studies community have defined lethality and combat power historically. As the Napoleonic adage goes, "in war, the moral is to the physical as three is to one." Yes, warrior ethos matters, but it animates a force that is part of society and nested within complex civil-military institutions, and whose combat power is linked to training, education, and the path dependence of weapons procurement. In other words, understanding lethality cannot be reduced to warrior ethos alone. It requires a spirited debate about how to close with the enemy on a modern battlefield littered with sensors and drones, where nimble minds capable of constant adaptation regardless of gender are likely more important than brute physical strength.

Q1: Why is "lethality" at the center of U.S. defense debates today?

A1: Secretary Hegseth has made lethality the centerpiece of his defense agenda, framing it largely in physical terms and about realigning warrior ethos in the wake of what many, not just Hegseth, see as an overreach during the Biden presidency to bring social debates into the uniformed ranks.

The secretary of war's passion and interest in warrior ethos echoes an enduring set of ideas about war, most famously the French concept of élan. Rooted in Henri Bergson's idea of élan vital from Creative Evolution (1907), this was not just military rhetoric but part of a broader philosophical revolt against modern views of life. Bergson argued that human action was animated by a vital impulse-an intuitive spirit of freedom and creativity beyond rational calculation. French military theorists adopted this vitalism to describe élan as the fighting spirit that could animate armies, offsetting material deficiencies and giving war a transcendent moral character.

After France's defeat in the Franco-Prussian War, a generation of officers embraced élan vital as a philosophy of war. It fit their mood of revanchism and their desire to break free from perceived constraints, merging national pride, masculine ethos, and military hierarchy with Bergson's idea of a vital spirit. In many ways, Hegseth's emphasis on the "warrior ethos" reflects a similar impulse: a veteran's reaction to the long wars in Iraq and Afghanistan and the belief that American warriors were held back, their fighting spirit restrained by bureaucracy and overmanagement.

Q2: How has lethality been understood historically?

A2: Despite famous quotes like Stalin's "quantity has a quality all its own," most military theory does not reduce lethality and combat power to mass alone. Secretary Hegseth is not alone in seeing a central role for moral components in generating a more lethal fighting force. In On War, nineteenth-century Prussian soldier and theorist Carl von Clausewitz explored both the physical, available means (i.e., troops, weapons, and logistics) and moral components that define combat. According to Clausewitz, the "art of war has to deal with living and with moral forces." This characterization continues to the present. British military doctrine defines fighting power as conceptual (i.e., philosophy and doctrine), moral (i.e., motivation, cohesion, and ethical foundation), and physical. It builds on Clausewitz and work by Martin Van Creveld, whose study of why the German army outperformed Allied formations tactically highlighted the role of cohesion and morale, decentralization (i.e., auftragstaktik), professionalism, training, and organizational culture.

A separate school of thought sees warfighting and competitive advantage emerging from new technology. This approach, often called offsets, charts changing military power in terms of new epochs-revolutions in military affairs-and even larger, military revolutions that transform all of society.

Yet even technology alone is insufficient to explain lethality and combat power. Andrew Marshall charted how the very structure of the national security state skewed lethality. In The Pursuit of Power, historian William McNeill showed that new weapons-from muskets to machine guns-only mattered when governments and societies could actually organize and train forces to use them well. Technological advances only matter if societies can organize and employ them effectively. History is littered with crucial cases illustrating that integration and competence, not weapons alone, determine outcomes. Lethality and combat power are about more than just technological offsets. For researchers like Stephen Biddle, entire concepts of force employment define what constitutes military power.

Q3: How is lethality changing in the twenty-first century?

A3: The modern battlefield doesn't look the same as it did 30 years ago. While there certainly are enduring aspects like warrior ethos, conflict today is increasingly a joint all-domain contest of wills that involves a mix of cyber operations, electronic attack, psychological warfare, and multidomain operations. This approach prefaces information and battle networks. In fact, observers in China see these changes emerging from the U.S. victory in the Gulf War and accelerating with the emergence of AI and concepts like mosaic warfare.

In modern war, the most lethal forces are those that can

• sense the environment through diverse and resilient intelligence, surveillance, and reconnaissance, including collecting open-source and publicly available information;
• orient and decide what targets to strike faster than the enemy, even under cyber and electronic attack that degrade command and control;
• use a "symphony of capabilities" to coordinate effects across multiple domains in pulsed strikes;
• preserve cohesion and judgment while the network is under attack in space and cyberspace and the adversary is launching deepfake attacks; and
• integrate drones swarming the frontlines with operational art.

The kill chain is no longer linear. It is a contested, adaptive web-a complex system where the side that adapts faster gains tempo and advantage. As a result, war is a contest of wills, a human endeavor, and a contest of competing battle networks. Winning, therefore, requires that investments in both warrior ethos and the types of modular, open systems soldiers need to adapt on the frontline faster than the enemy. And it means finding where it makes sense to substitute drones for élan.

Q4: What does this mean for U.S. defense policy?

A4: A clear concept of what constitutes lethality in modern war should be at the heart of U.S. defense policy. Combined arms is giving way to combined effects and creating options to fight adversaries in multiple domains. The definition of lethality must evolve as the character of war changes. It should include, per the secretary of war, elements that focus on how to build cohesion and deepen warrior ethos. But it should also include investments and guidance that prioritize how to build a force built around more drones than people and attacking adversaries in multiple domains. And it must include transformational investments in AI that redefine everything from how military organizations plan and execute combat operations to training and educating future warriors. Last, it must be tailorable to address pending changes in the National Defense Strategy and the range of war plans the nation sees as necessary to secure U.S. interests.

Benjamin Jensen is the director of the Futures Lab at the Center for Strategic and International Studies in Washington, D.C.

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