Ukraine War Rewrites Nuclear Age Combat Rules

In September 2022, when Vladimir Putin threatened to use "all weapon systems available," US intelligence assessed the likelihood of actual nuclear use as a "coin flip." Ukrainian forces had just achieved breakthrough success in Kharkiv, and the potential collapse of Russian lines could have opened a path to Crimea—threatening Putin's regime itself.

That moment shattered a 30-year post-Cold War illusion. Nuclear weapons are no longer politically "unthinkable." The Ukraine war demonstrates that nuclear threats are real and fundamentally reshaping how wars are fought in the 21st century.

Rebecca Lissner and John Kawika Warden's analysis in Foreign Affairs identifies four critical lessons the United States must absorb from this conflict—lessons that could determine success or failure in future great-power confrontations.

Nuclear Threats Aren't Empty Bluster

The first lesson shatters decades of complacency: the risk of nuclear weapons use is genuine and cannot be dismissed. For years after the Cold War, American policymakers treated nuclear weapons as largely irrelevant to conventional warfare. The durability of non-use since 1945 fostered dangerous overconfidence.

Russia's nuclear saber-rattling throughout the invasion has been persistent. Most was bluster, but fall 2022 was different. Ukraine's counteroffensive had made significant gains, seemingly catching Russian forces off guard. The catastrophic collapse of Russian lines appeared plausible, potentially producing a cascading failure that could threaten Putin's survival.

Putin's September declaration was his most direct nuclear threat: Russia would "certainly make use of all weapon systems available" to defend its territorial integrity, adding this was "not a bluff." US intelligence corroborated the warning's seriousness. CIA Director William Burns later publicly confirmed a "genuine risk" of nuclear use if Russian army lines collapsed. National Security Adviser Jake Sullivan characterized the likelihood as a "coin flip."

The implications extend beyond Russia. North Korea is building a diverse arsenal with weapons explicitly designed for tactical use, and Kim Jong Un has signaled he wouldn't hesitate to use them in a Korean Peninsula conflict. China's long-standing no-first-use pledge likely makes its nuclear threshold higher, but whether that policy would hold if Beijing faced defeat over Taiwan remains uncertain—especially as China develops a larger, more diverse arsenal.

Conventional War Persists Under Nuclear Shadow

The second lesson is paradoxical: even under nuclear threat, protracted and highly destructive conventional war remains possible. Despite Russia's overwhelming nuclear advantage, Ukraine hasn't capitulated. Instead, it has denied Moscow its primary war aims.

Ukraine repelled the initial invasion, maintained sovereign independence, and inflicted enormous losses—over one million Russian casualties and billions in destroyed equipment. Rather than submit to nuclear coercion, Ukraine has gradually escalated its own operations.

Most remarkably, Ukrainian forces have struck targets inside Russia that many analysts once viewed as likely nuclear redlines: oil and gas infrastructure, logistics hubs, the Kerch Strait Bridge connecting Crimea to mainland Russia, and even Russia's nuclear-capable strategic bomber fleet. Ukrainian forces have also conducted incursions into Russian territory itself.

This demonstrates nuclear weapons' limitations as coercive tools. Nuclear threats alone cannot compel submission from a determined adversary—a sobering lesson for understanding future conflicts.

Escalation Thresholds Aren't Fixed

The third lesson challenges conventional strategic thinking: escalation thresholds aren't predetermined but emerge through ongoing contestation during war. What constitutes a "red line" evolves dynamically through conflict.

Early in the war, Western restraint was extreme. Germany hesitated to provide even 5,000 helmets. The US restricted HIMARS rocket ranges. But these "red lines" continuously shifted. The West gradually provided more powerful weapons, and Ukraine expanded its strike range deeper into Russian territory.

This pattern suggests that initial restraint in future conflicts may not persist. Escalation management isn't governed by fixed rules but represents a dynamic process of tacit bargaining between adversaries. Both sides continuously test boundaries, and those boundaries evolve based on responses and changing battlefield conditions.

Allied Friction Is Inevitable

The fourth lesson acknowledges an uncomfortable reality: friction with allies over risk tolerance and escalation management is unavoidable. Throughout the Ukraine war, tensions have emerged between the US and European allies over support levels and timing.

Some allies pushed for more aggressive assistance while others worried about escalation risks. These differences are natural—countries have different geographic positions, historical experiences, and domestic political constraints. The challenge isn't eliminating such friction but managing it effectively.

This has profound implications for US defense planning. In conflicts involving Taiwan or the Korean Peninsula, America will likely face similar allied disagreements. Some partners may be more risk-tolerant, others more cautious. The US needs mechanisms for managing these differences before conflicts begin, not during them.

Preparing for Limited Nuclear-Age Wars

These lessons point toward a critical strategic imperative: the United States must intensively plan and prepare for limited war with nuclear-armed adversaries. This represents a fundamental shift from post-Cold War assumptions.

Washington needs updated policies and defense planning for limited conflicts, ensuring flexibility to wage—and win—21st-century wars. This includes refreshing assessments of how Ukraine may have altered adversaries' nuclear strategies and thresholds. It means expanding credible military response options, including offensive cyber, space, and advanced conventional capabilities.

Crucially, the US cannot do this alone. It must coordinate closely with prospective coalition partners before conflicts begin, even while acknowledging perfect alignment is impossible. This makes security guarantees more credible and, should deterrence fail, escalation management more effective.

The Stakes of Learning

The Ukraine war represents the first conflict since the Soviet Union's fall where two major nuclear powers have found themselves on opposing sides of high-stakes hostilities. The strategic lessons emerging from this confrontation will shape warfare for decades.

If Washington fails to absorb these lessons, it will find itself dangerously ill-equipped for great-power war at precisely the moment when such conflicts are becoming more likely. The window for preparation is narrowing as tensions rise across multiple theaters—from the Taiwan Strait to the Korean Peninsula to potential NATO-Russia flashpoints.

The war's resolution remains uncertain, but its strategic implications are already clear. Nuclear weapons haven't eliminated conventional war—they've made it more complex, requiring more sophisticated escalation management and alliance coordination.