Ukraine’s invasion of Kursk is not just a brazen attempt to stave off Russian invasion, but also marks the first time a nuclear power has faced invasion and occupation by another country.

As stated in an analysis by Wall Street Journalfor decades, nuclear de-escalation theory assumed that countries with nuclear weapons were largely immune to attack because the aggressor risked detonating a Armageddon. Relatively small states, including Israel, Iran, North Korea and Libya, have sought to equip themselves with nuclear weapons in part to deter attacks by larger, better-equipped adversaries.

In particular, the threat of annihilation has protected nuclear-armed countries from large-scale attacks and kept the peace.

Ukraine is not a nuclear power and yet Kiev has managed for more than three weeks to control almost 500 square miles of territory. This is a major turnaround.

Western leaders, military analysts and nuclear theorists are pondering what this development means for the prospect of Russian escalation, but also for future war games.

Russia’s nuclear doctrine states that Moscow will only resort to nuclear weapons if its sovereignty or territorial integrity is threatened country. Although Ukraine occupies a sliver of Russian territory, neither side appears to consider the Kursk region strategically vital, so Ukraine’s offensive—however damaging to the Kremlin’s image—shows no sign of crossing a Russian red line.

But ambiguity and uncertainty are part and parcel of nuclear craftsmanship, the WSJ notes.

“Nobody really knows the Russian red line – it’s never been explained precisely,” said Nikolai Shokov, a former Soviet and Russian arms control negotiator. “We may find out later that we crossed the red line two months ago,” said Shokov, who now briefs Western military leaders on Russian strategic thinking.

An argument that could possibly be invoked by Vladimir Putin, notes Shokov, is to view threats to his regime as overarching threats to Russia. Seen through that lens, significant Ukrainian gains or Russian losses could trigger a nuclear escalation — though it would likely begin with greater use of non-nuclear weapons, not a surprise attack, Shokov said.

Fear of crossing Russian red lines has shaped President Biden’s approach to war. Not wanting the war in Ukraine to turn into a direct battle with the nato, he hesitated to give Ukraine weapons it requested, including tanks, advanced missiles and jet fighters. Kiev eventually took most of them, prompting the Ukrainians and their Western allies to argue that Putin’s red lines were flexible.

Ukraine intends to show with its invasion of Kursk that this taboo can be broken without dire consequences. Part of the goal is to convince the White House that Ukraine should be allowed to use more lethal and accurate US weapons to attack Russia.

Many Western officials, particularly in Washington and Berlin, remain more cautious because Putin is unpredictable.

Uncertainty about where Russia’s red lines lie is “the fundamental challenge of strategic ambiguity,” said Janice Gross Stein, a professor of conflict management at the University of Toronto.

The events caused by Ukraine’s invasion of Kursk harken back to the Cold War. When the Soviets developed an atomic bomb in 1949, four years after the US, Western strategists struggled to envision how the fearsome weapons could be brought into battle.

Their enormous destructive power—and far greater danger than thermonuclear weapons—has led to the creation of a new field of nuclear deterrence thinking and the analytical tools to support it.

Nuclear strategy and defining an opponent’s redlines remains a high-stakes game.

Putin has more or less directly threatened to use nuclear weapons since launching his full-scale invasion of Ukraine in 2022. But a decade ago, Russia’s seizure of Ukraine’s Crimean peninsula signaled a new level of post-Cold War aggression. .