Russia's invasion of Ukraine drew rebukes and condemnation from leaders around the globe, including an alliance that is central to the ongoing conflict.
NATO Secretary-General Jens Stoltenberg called on Russia to withdraw from Ukraine after President Vladimir Putin directed troops to begin an incursion that had been brewing for months.
"We fully support Ukraine’s sovereignty and territorial integrity and Ukraine’s right of self-defense," Stoltenberg said in a prepared statement.
Ukraine, a post-Soviet state, is not a member of NATO, but it has drifted in recent years toward the West. In 2008, NATO said it would eventually accept the eastern European country into the alliance, a promise Putin denounced at the time and has worked against since.
Geography of eastern Europe: Where is Ukraine? Where are NATO members? A guide
Russia now demands that Ukraine never be allowed to join the alliance and that the United States largely withdraw from Europe. It's the most significant challenge to the military alliance's power since the end of the Cold War.
At issue in the conflict is a debate over the identity and expansiveness of the military alliance, as well as the broader structure of European security.
Here's what to know about NATO and what the alliance means in the current conflict:
The North Atlantic Treaty Organization was created in 1949, with Europe still reeling from World War II devastation. The alliance was created by the U.S., Canada and 10 other nations as a system of collective defense against the Soviet Union.
Its basic principle is that mutual defense is in the group's best interest. That is the centerpiece of NATO's founding treaty – signed by President Harry Truman – enshrined in the Article 5 provision that requires member states to come to the aid of their allies in the event of an attack.
The provision has been invoked only once, when the U.S. called on the alliance after the Sept. 11, 2001 terror attacks. NATO has notable military interventions in the Balkan countries in the 1990s and 2000s, as well as more afield interventions like in Libya and Somalia.
NATO has grown to 30 member states that, according to the organization’s website, are committed to “guarantee the freedom and security of its members through political and military means.”
Historians have credited NATO with unifying Western democracies during the Cold War, but the organization has faced internal struggles. France formally pulled out of the group’s military command in the 1960s, angering the U.S., and then rejoined in 2009.
The present crisis has again surfaced some of those tensions.
On Tuesday, Croatia said it would withdraw its forces from NATO deployment in eastern Europe should a conflict arise. While NATO members, like the Baltic states, Poland and Romania have taken a hawkish stance against Russia, countries like France and Germany are still stressing peace above all else.
Visual explainer: How US and its allies could respond to Russian invasion of Ukraine
The divisions put into question the bloc's "credible commitment" to conflict, essentially meaning whether other countries believe the alliance will actually follow through on its commitment to fight if it's tested.
President Joe Biden, an ardent believer in the liberal international order, has said the U.S. will follow through on its commitments in NATO should it be required.
"We're going to actually increase troop presence in Poland, in Romania," Biden has said when pressed on how NATO would respond to an invasion of Ukraine, citing the "sacred obligation in Article 5 to defend those countries."
Biden added that while the U.S. isn't obligated to defend Ukraine with its own military "we have great concern about what happens in Ukraine."
Follow Matthew Brown online @mrbrownsir.
This article originally appeared on USA TODAY: What is NATO? In Ukraine-Russia crisis, alliance is at forefront