What is NATO, and how has it responded to Russia’s invasion of Ukraine?

By Saturday, Kyiv was transforming into a war zone. Nearly 200 Ukrainians have been killed in fighting across the country, with more than 1,000 wounded, according to Ukraine’s health minister. And 100,000 have fled to Poland alone.

Stoltenberg on Friday announced that NATO would deploy troops from an alliance response force for the first time to shore up defenses in Eastern Europe.

NATO lays out its next steps after Russia’s ‘brutal act of war’ on Ukraine

Ukraine is not a member of NATO, the military alliance of mainly Western countries united by a mutual defense treaty. But post-Cold War tension between the West and Russia over NATO — or the North Atlantic Treaty Organization — is at the heart of the current crisis.

Since 1999, 14 nations have joined NATO, including Hungary, Poland, Bulgaria and the Baltic states. Russia has demanded that the alliance stop expanding eastward — and bar Ukraine from joining. Ukraine’s government has said that it would like to enter the alliance, along with other nations that were once part of or allied with the former Soviet Union.

In remarks on Thursday, President Biden vowed that the United States would meet its commitments under Article 5 of the NATO treaty, “which says that an attack on one is an attack on all.”

“The United States will defend every inch of NATO territory with the full force of American power,” he said.

While Stoltenberg on Thursday reiterated his support for Ukrainian sovereignty, he emphasized that there “are no NATO combat troops inside Ukraine at all … [and] we do not have any plans and intentional deploying NATO troops to Ukraine.”

Here is some essential background about NATO and Article 5. The collective defense clause was invoked by NATO members only once, following the attacks in the United States on Sept. 11, 2001.

Military trainers, missiles and over 200,000 pounds of lethal aid: What NATO members have sent Ukraine so far

What to know