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 3 million personnel are owned and brought to the range by the 28 member states, mostly at their own cost. The only military equipment NATO has is a fleet of early warning radar planes and, from next year, five surveillance drones.
Here's a look at how NATO works and why it matters:
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. Montenegro is set to join soon. 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.
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 1 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 28 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 Sept. 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.
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