Nearly three years after Russia’s massive invasion of Ukraine, the Biden administration gave Ukraine the green light to invade deeper into Russia using longer-range US-supplied missiles.

The Ukrainian army quickly took advantage of that permission: On Tuesday it attacked an arms depot about 70 miles from the Ukrainian border.

US and NATO allies have hesitated to supply advanced weapons, such as the Army Tactical Missile System (ATACMS) used by Ukraine in that attack, for fear of Russian retaliation against NATO sites – or even nuclear escalation.

Russian President Vladimir Putin stoked the specter on Tuesday by signing a new version of the country’s nuclear doctrine, which in theory would make it easier for Russia to use nuclear weapons in the conflict. The new doctrine specifically allows a nuclear strike in response to an attack with conventional weapons – such as the longer-range missiles that Ukraine is now allowed to use – if those attacks involve “the participation or support of a nuclear force,” likely referring to the US and others NATO countries.

Throughout the war, Russian leaders threatened to use the country’s nuclear weapons in the conflict if they believed it was necessary. That has led Ukraine’s allies to be cautious about the amount of aid they have offered and led to restrictions on what Ukrainian forces can do with those weapons.

Ukraine’s new ability to use longer-range missiles to hit Russian territory – and Putin’s confirmation of new nuclear rules – have once again raised the question: Could Russia’s war in Ukraine escalate into a nuclear conflict?

Throughout the war, experts downplayed Russia’s appetite for nuclear conflict. But the rapid escalation of the conflict in recent weeks, and especially Russia’s new nuclear doctrine, could mean this possibility is closer than before.

What are the chances that Ukraine’s new missile capabilities will lead to nuclear escalation?

Since the beginning of Russia’s invasion of Ukraine, Putin and other officials have made statements, both explicit and indirect, that Russia could be pressured to use nuclear weapons. Most experts agree that the risk of Russia using such weapons is small, but not negligible.

An earlier version of the doctrine that Russia updated on Tuesday said the country would tap its nuclear arsenal only under four circumstances: receiving credible data about a ballistic missile attack; nuclear or other weapons of mass destruction against Russia or its allies; attacks on Russia’s nuclear infrastructure; or attacks with conventional weapons that threaten “the very existence” of the Russian state. Under the new doctrine, Kremlin spokesman Dmitry Peskov indicated that attacks like Ukraine’s on Tuesday could trigger a nuclear response.

However, that change should not be interpreted as a response to the Ukrainian strike, Samuel Charap, professor of Russia and Eurasia policy and senior political scientist at the RAND Corporation, told Vox.

The new doctrine “has been in the making for a long time,” Charap said. “The timing may be related to the attack on the decision, but the substance has been brewing for some time.”

Putin and other officials have, it appears, made no major preparations to actually use nuclear weapons. However, as the war continues, the threats from Putin and his officials have become clearer and involve the actual demonstration of nuclear capability. As recently as July, Russia and Belarus held joint military exercises that demonstrated Russia’s tactical nuclear capabilities.

Initially, the US and NATO reacted cautiously to Russian threats by denying Ukraine weapons or imposing restrictions on their use. But over the past three years, Western countries have given Ukraine access to increasingly sophisticated weapons systems as little has come of Putin’s threats of nuclear war and war with NATO.

In addition to the risk of nuclear war, Ukraine’s allies had to consider concerns that they might be more directly involved in the conflict. Although Ukraine is not a member of the NATO military alliance, Putin has previously warned that permission to use longer-range missiles (like those used on Tuesday) in Russia would be considered a NATO attack on Russia.

U.S. officials who spoke to The Associated Press said they had expected a response from Russia, but that Russian officials’ warnings were seen as inflammatory rhetoric and would not provoke any change in U.S. action. That suggests that the US does not believe there is much danger in the short term if Russia uses its nuclear capabilities.

Russia “has never explicitly, at an official level, warned that they would use nuclear weapons in response to X, Y or Z,” Charap said. “The only explicit red line they ever drew was the use of US (or) Western long-range weapons to attack Russia which has now been crossed. So I can imagine that people expect their answer, and it won’t just be in words.”

After a thousand days of war, the conflict appears to be approaching an impasse. There is no clear path to decisive victory for either side. Each side is using new tactics to gain advantage: Ukraine with longer-range missiles; Russia with recruits from North Korea. And now Russia and the US appear to be simply responding to each other’s escalations: Russia put North Korean troops on the battlefield, the US responded by authorizing the use of longer-range missiles, and Russia abandoned its new nuclear stance. That kind of behavior is both reckless and dangerous, Charap said.

“You’re in a spiral, which is the definition of a tit-for-tat dynamic, where your actions are driven not by your goals, but by counteracting what the other person is doing,” Charap said. “That only goes in one direction: continuing to raise the bar. The spiral dynamic just continues until someone gets out of hand or someone decides to end it.”