As Russia began its invasion of Ukraine, some U.S. officials and analysts were already warning of a possible escalation of the conflict into a wider war.
The trigger: Article 5 of the North Atlantic Treaty.
As the fighting continues, we unpack what Article 5 is, what could invoke it, and the places and people to watch.
What is it?
Article 5 is a key pillar of the 1949 North Atlantic Treaty — also known as the Washington Treaty — and is based on the principle of collective defense. It means that an attack on one member of NATO is deemed to be an attack on all.
Ukraine is not a member of NATO, but a number of its neighbors are.
"There are 30 members of NATO, and we have given all of them Article 5," said Mary Elise Sarotte, a professor of history at Johns Hopkins University and author of Not One Inch: America, Russia, and the Making of Post-Cold War Stalemate.
"And I don't think Americans understand this — that means we are obligated by treaty to basically defend them or go to war if they are attacked by Russia. We are obliged to treat an attack on Estonia as if it were an attack on Chicago by treaty."
"So, if there is an Article 5 incursion, this could very quickly become not Ukraine's war, but our war."
The article states:
[NATO members] will assist the party or parties so attacked by taking forthwith, individually and in concert with the other parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Article 5 has been invoked once — in response to the 9/11 attacks.
What could trigger this?
Russia has attacked Ukraine by land, sea and air – but there remains a question over cyberattacks.
Less than 24 hours after the invasion began, U.S. Senate Intelligence Committee chairman Mark Warner, D-Va., warned that if Russia launched major cyberattacks on Ukraine, the damage could spill beyond the country's borders.
"When you launch cyberattacks, they don't recognize geographic boundaries. Some of that cyberattack could actually start shutting down systems in eastern Poland," he said.
"If you shut down Polish hospitals because they can't get power to take care of their people, you're rapidly approaching what could be viewed as an Article 5 violation of NATO ... so we are in an uncharted territory."
Retired U.S. Navy Adm. James Foggo echoed this sentiment shortly after the attack began, saying the U.S. was right to station troops in Poland.
"We're doing that because we do not want the spillover effects to impact any of our NATO allies," he said. "If there is an interaction — a kinetic interaction, a fight — between Russian troops and American troops it could trigger a world war. So, we need to reinforce our allies and make sure that this crisis is contained just to Ukraine and continue to support Ukraine with lethal weapons."
The countries that could be affected
It's unclear whether a traditional armed conflict will go beyond Ukraine and Russia and trigger it again. But Sarotte said there were particular places to watch.
"We're going to see, I think, regime change and we're also going to see [Russian President Vladimir Putin] take control of more areas. I think he's de facto already taken control of Belarus," she said.
"Then there is a question about does he try to do something to the Baltics, which also used to be part of the former Soviet Union. And that's really dangerous because the Baltics are in NATO."
On Friday, NATO leaders issued a scathing condemnation of Russia after meeting to discuss the invasion of Ukraine, calling the attack "the gravest threat to Euro-Atlantic security in decades."
The alliance said it was now taking "significant additional defensive deployments of forces" to its eastern flank and said it would give "political and practical support" to Ukraine.
In a statement on Thursday, it said: "Our commitment to Article 5 of the Washington Treaty is iron-clad. We stand united to protect and defend all allies."
Alejandra Marquez Janse and Ayesha Rascoe contributed to this report.