(CNN) -- On April 4, 1949, 12 nations signed a treaty in Washington, D.C., creating a defense alliance known as the North Atlantic Treaty Organization.

In the 50 years since, NATO has added seven members -- including three former members of its one-time communist counterpart, the Warsaw Pact -- and signed agreements with other countries aimed at promoting and sustaining peace in Europe.

But with the fall of the Soviet Union and communist regimes elsewhere in recent years, the alliance's role has shifted from defense to that of a regional police force. The bloodshed in Bosnia and Kosovo has led NATO to intervene not just to keep the peace, but to impose it.

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The armed forces of all member nations are at the disposal of NATO should the need arise. Commanders at NATO determine what personnel and material is needed in each case and request it from the members.

Charles Heyman, editor of Jane's World Armies, says the decline in defense budgets around the world has cut deeply into NATO's standing forces and made gearing up for incidents like the war in Kosovo more difficult.

"You would expect each of those nations to have a small number of forces standing by," Heyman says. "But it changes all the time."

In general, NATO's key units for military response are called the Allied Rapid Reaction Corps (ARRC), which can be mobilized in seven to 15 days.

Included in the ARRC forces are national divisions, such as the British armored division, the Turkish mechanized division and the Spanish Rapid Action Force, which are currently assigned to NATO.

There are also "framework divisions," where one nation leads and others contribute, and multinational divisions, in which the member nations contribute command, staff and combat forces in equal shares.

The units range in size depending on their duties, but Heyman estimates the average division at 12,000 soldiers.

"In that figure, between 3,000 and 4,000 are fighting troops," Heyman says. "The rest are bottle washers, engineers, technicians."

The current ARRC commander, and the commanding field officer in the Kosovo conflict, is British Gen. Sir Michael Jackson. Jackson takes his orders from U.S. Gen. Wesley Clark, NATO's Supreme Allied Commander in Europe.

Jackson's deputy is an Italian general, and the chief of staff is another British general. About half of the 1,000 people at the ARRC headquarters in Rheindahlen, Germany, are British.