BRUSSELS -- NATO has gone into combat for the first time since it was founded almost 45 years ago, with U.S. fighters under allied command shooting down four Serb warplanes over Bosnia on Monday.


Founded in 1949 to protect the West from Soviet military power, the North Atlantic Treaty Organization never fired a shot in anger during the decades of the Cold War.


The 16-nation Western alliance, which had a purely defensive mandate, did not intervene when Soviet tanks rolled in to crush the uprisings in Hungary in 1956 and in Czechoslovakia in 1968.


Following the end of the Cold War, NATO sought to forge a new role for itself in Europe which would make use of the alliance's highly developed military structure.


After much debate, the allies finally agreed that NATO could be used beyond the simple defense of its member states, in support of the United Nations for missions such as peacekeeping.


When the allied coalition went to war against Iraq in 1991, it was under a UN flag rather than NATO command.


But, with the UN increasingly stretched with peacekeeping operations around the world, NATO offered its help in the conflict in former Yugoslavia.


In 1992, the UN ordered a ban on military flights over Bosnia to prevent the warring parties from bombing and strafing ground targets.


NATO began monitoring compliance with the ban in October 1992, using surveillance planes.


In April 1993, NATO fighters began enforcing the ban -- the first time that the alliance had deployed forces outside the borders of its member states.


Although there have been some violations of the UN ban on flights, mostly by helicopters, NATO has taken no action until now.


In June 1993, the alliance agreed to offer more planes to protect UN peacekeepers on the ground in Bosnia. They have not been called upon so far for air strikes.


NATO warships have also been enforcing a naval blockade in the Adriatic, making sure that UN economic sanctions against Serbia and Montenegro are respected.


NATO threatened to order air strikes around Sarajevo last August but never carried it out.


Following the mortar attack on a crowded Sarajevo marketplace which killed 68 people earlier this month, NATO gave the Bosnian Serbs and Moslems 10 days in which to withdraw their big guns from around the Bosnian capital or hand them over to UN control.


The deadline ran out on Feb. 20 and NATO and the UN said there had been almost complete compliance and so no air strikes were ordered.


On Monday, however, after six Yugoslav-built G-4 Galeb light attack planes twice ignored warnings to leave the UN's no-fly zone over Bosnia, U.S. F-16 fighters opened fire on them with AIM heat-seeking missiles.