“We’re already in [World War III]. We have been for some time,” Fiona Hill, former director of the U.S. National Security Council, recently said in an interview with Politico on Monday, outlining the current state of the world. 

“Sadly, we are treading back through old historical patterns that we said that we would never permit to happen again,” Hill added. 

For now at least, the fighting is limited to Ukraine, and has yet to spill over into nearby NATO member countries. The U.S. and its allies, for their part, have condemned Putin’s actions while refusing to send their own troops to Ukraine, signaling an effort to avoid expanding the conflict. 

But experts warn that war is never predictable. Russia is a nuclear state, which magnifies every problem. 

Inadvertent escalation

Currently, the war is being fought in the eastern, southern, and northern regions of Ukraine, in cities including Kherson, which was recently claimed by Russian forces. Meanwhile, a large convoy of Russian military vehicles appears to have stalled 19 miles outside of the capital Kiev.

But the more the Ukraine crisis drags on, the bigger the risk of an “inadvertent escalation,” or something that goes wrong on the margins of the war, could cause it to expand. 

“The bigger the conflict, the greater the possibility of something like that happening,” Mary Elise Sarotte, a post-Cold War historian and author of the 2021 book Not One Inch: America, Russia, and the Making of Post-Cold War Stalemate, told Fortune.

During the Cold War, the U.S. and the Soviet Union approached nuclear war several times. And many of these close calls were because of individual mistakes and human error.

“A dangerous and tragic case would be if Russian forces were to inadvertently, and I want to emphasize inadvertently, launch a missile that landed in a bordering NATO country, such as Poland,” Glennys Young, Russian studies expert and chair of the University of Washington’s history department, told Fortune. 

“If this were perceived by NATO commanders as an attack, and hopefully it wouldn’t, this would trigger the provisions of the NATO alliance’s Article Five,” she continued.

NATO’s Article Five emphasizes “collective defense,” the idea that an attack on one NATO-allied country constitutes an attack on all member nations, theoretically provoking a mass, global response. A raging war in Ukraine, which shares borders with four NATO countries (Romania, Hungary, Slovakia, and Poland), raises the risk of Article Five being invoked. Doing so would involve deliberation from all NATO members and, potentially, Russia, and wouldn’t necessarily translate to an immediate response.

“There’s this phrase, ‘the fog of war,’” Young said. “It means that even though one often has the sense that military maneuvers, campaigns, and attacks are orchestrated, one can never know exactly how they’re going to play out.”

Putin’s personality

Putin’s decision to invade Ukraine was immediately met with international alarm, and some felt that his actions could be the biggest factor in creating a more global crisis.

“[Putin] has been doing so many things recently that are just brazen, reckless, unpredictable, and frankly self-harming,” Sarotte said. “Given that a person like that is now in charge of a nuclear arsenal, I do think there is a serious concern for war.”

Some longtime Russia observers have been surprised by Putin’s determined stance on Ukraine. They say he’s preoccupied with how the end of the Cold War turned out.

“His unstated goal is to avenge what he has called the greatest tragedy of the 20th century: The collapse of the Soviet Union, the unraveling of the Soviet empire and the territories that it once controlled,” Young said. 

Putin has been undeterred by the sanctions President Joe Biden and other Western leaders have imposed on Russia while he’s doubled down on his invasion.

“Sadly, that seems to be the pattern we’re seeing. The greater the resistance in Ukraine, the more Putin seems to be willing to use intensified military force,” Young said, mentioning the recent civilian bombing of Kharkiv, Ukraine’s second largest city, which killed 25 people and injured 112 more as of Wednesday.

Russia’s isolation

Putin has been relatively isolated throughout his Ukraine campaign. His main ally is Alexandr Lukashenko, president of Belarus, where tens of thousands of Russian troops have been stationed since before the Ukraine invasion began. 

Some countries with close ties to Russia were initially neutral in their reaction to the invasion, with China neither criticizing nor endorsing Putin’s actions. But on Wednesday, six days after the invasion began, even China acquiesced and called the invasion of Ukraine a war, with officials saying they were “extremely concerned” with how Ukrainian civilians were being treated, indicating Beijing’s desire to prevent further escalation.

“I don’t think China and India are going to pick sides any more than they have,” Paul D’Anieri, a political science professor at the University of California at Riverside and author of the 2019 book Ukraine and Russia: From Civilized Divorce to Uncivil War, told Fortune. 

“I don’t think either of them really wants to alienate Russia, but nor does either country have any reason to support what Russia is doing,” D’Anieri continued.

Other countries publicly supporting Ukraine, including the Netherlands, Sweden, Finland, and Germany, have all sent missiles, rocket launchers, or machine guns to help with its resistance. If the conflict grows, western governments may be more inclined to respond directly, but that appears to be a long way off, according to D’Anieri.

“If violence becomes more indiscriminate, and lots more innocent people are killed, I think you’ll see more outrage, more willingness to help. But I think we’re still a very, very long way from western governments wanting to put their troops on the ground in Ukraine,” he said.

Senior ministers have dismissed talk that Russia could escalate the conflict with nuclear weapons, accusing Western politicians of intentionally fanning political flames. “It is in the heads of Western politicians that nuclear war is always revolving, and not in the head of the Russians,” Russian Foreign Minister Sergei Lavrov said on Thursday. 

But Lavrov also indicated that Russia would be prepared to retaliate to any signs of aggression, warning Western governments to stand back. “If a real war against us starts developing, those who have such plans must have a think, and I believe such plans are being carried out,” he said.

The U.S. has said it will not send troops to Ukraine, preferring instead to rely on diplomacy and building an international consensus to condemn Putin. But even if western nations stand back, any Russian success in Ukraine would create more global tension. 

“There are some places now where NATO and Russia share a border, but they’re relatively limited,” D’Anieri said. But, he added, if Russia successfully takes Ukraine, “you’d be back to a situation where you had a very long border between Russian-controlled territory and NATO.”

If this happens, D’Anieri believes tensions between Russia and the West would rise to their highest point since the 1950s. 

As for Sarotte, she said the situation is among the most dangerous in recent memory, and is still rapidly evolving. “If you had asked me two weeks ago what the chance of major nuclear conflict was, I would have said pretty low, but now I would say I don’t know, and that’s not good,” she said.