What is NATO? It sounds strange to characterize the North Atlantic Treaty Organization as a huge gun club, but the comparison can be useful in understanding the world's biggest military alliance.
Like many gun clubs, NATO actually has no weapons of its own. The battleships, war planes, missiles and potential pool of more than three million personnel are owned and brought to the range by the 30 member states, mostly at their own cost. The only military equipment NATO has is a fleet of early warning radar planes and surveillance drones.
Here's a look at how NATO works and what is its role in the Ukraine crisis:
What is it?
This club, with main headquarters in Brussels and military HQ in Mons, Belgium, is open to any European nation that wants to join and can meet the requirements and obligations. Bosnia-Herzegovina, Georgia and Macedonia are waiting in line.
The Soviet Union, during the Cold War, and Russia now have been major preoccupations since the organization was founded in 1949, and in many ways remain NATO's reason for existing.
The United States is without doubt the biggest and most influential member. It spends more on its own military budget than all the others combined. It also pays just over 22 percent of NATO common funding for infrastructure and collectively owned equipment. So Washington has a big say in how things are run.
- Russia-Ukraine Live Updates: U.S. Shuts Airspace to Russian Aircraft
- Ukraine Crisis Shows U.S. 'Cannot Be Trusted,' Iran's Leader Says Amid Nuclear Talks
Smaller allies long to train and work with U.S. forces because it gives them access to equipment and expertise they cannot afford alone.
But NATO's decisions are made by consensus and there is no majority voting of any kind. This means that Albania, for example, has a veto just as final as Washington's.
Who's in charge?
The alliance's meetings – the North Atlantic Council – held at ambassadorial level almost weekly in Brussels, less often at the level of ministers or heads of state and government – are chaired by NATO Secretary General Jens Stoltenberg.
In essence, Stoltenberg runs the headquarters located near the Brussels airport, which is shifting just over the road this year to new premises being inaugurated by NATO leaders Thursday and estimated to have cost more than one billion euros.
He does not order the allies around. His job is to encourage consensus and speak on their behalf publicly as a single voice representing all 30 members.
Why should Americans care?
With the United States clearly able to take care of itself most of the time in military terms, many wonder why Americans should even care about NATO.
But the alliance is the one international forum where Washington agrees to put its military might up for negotiation and can be persuaded to act differently by its allies.
It's also an organization that uses plenty of U.S. taxpayer dollars. That money, in part, drives military spending and defense research and so provides plenty of jobs.
What does it do?
On the ground, NATO has notably helped to keep peace in the Balkans and combat the Taliban-led insurgency in war-torn Afghanistan - the alliance's biggest ever operation, launched after the United States triggered its "all for one and one for all" common defense clause in the wake of the September 11, 2001, attacks.
It is the only time the clause, known as Article 5, has been activated.
While the Soviet Union is long gone, NATO continues to see Russia as a security threat and to offer protection to concerned member states near Russia's borders.
NATO is now joining the international coalition fighting the Islamic State group and setting up a counter-terrorism intelligence cell to improve information-sharing. It will notably focus on so-called foreign fighters who travel from Europe to train or fight with extremists in Iraq and Syria.
What's NATO's role in the Ukraine crisis?
In 2004, pro-Russian candidate Viktor Yanukovich was declared Ukraine's president but allegations of vote-rigging trigger protests, known as the Orange Revolution, forcing a re-run of the vote. Pro-Western former prime minister, Viktor Yushchenko, is elected president.
Yushchenko takes power in 2005 with promises to lead Ukraine out of the Kremlin's orbit, toward NATO and the EU.
In 2008, NATO promises Ukraine it will one day join the alliance.
In 2013, Yanukovich's government suspends trade and association talks with the EU and opts to revive economic ties with Moscow, triggering months of mass rallies in Kyiv.
Ukraine's parliament votes to remove Yanukovich from power in February 2014, after bloodshed in the protests. Within days, armed men seize parliament in the Ukrainian region of Crimea and raise the Russian flag. Moscow later annexes the territory.
In 2019, former comic actor Volodymyr Zelenskyy was elected president. Zelenskiy appeals to U.S. president Joe Biden in 2021 to let Ukraine join NATO.
In the autumn of 2021, Russia started massing troops near Ukraine's borders in what it says are training exercises. Satellite images showed ongoing buildup of Russian forces near Ukraine.
In December, Russia presented security demands, including that NATO pull back troops and weapons from eastern Europe and bar Ukraine from ever joining.
In January, NATO put its forces on standby and reinforced eastern Europe with more ships and fighter jets. Washington responded to Russia's demands, repeating a commitment to NATO's "open-door" policy while offering a "pragmatic evaluation" of Moscow's concerns. Two days later Russia said its demands were not addressed.
In February, amid growing Western fears Russia could attack Ukraine, the United States said it would send 3,000 extra troops to NATO members Poland and Romania. Washington and allies say they will not send troops to Ukraine, but warn of severe economic sanctions if Russian President Vladimir Putin takes military action.
In a TV address on February 21, Putin said Ukraine is an integral part of Russian history and has a puppet regime managed by foreign powers. Putin orders what he called peacekeeping forces into two breakaway regions in eastern Ukraine, after recognizing them as independent.
Three days later Putin authorizes "special military operations" in Ukraine. Russian forces begin missile and artillery attacks, striking major Ukrainian cities including Kiev.
In response, Western allies announced new sanctions, with NATO sending additional forces to the region and NATO allies vowing to send arms and supplies to Ukraine.