On 21 November 1994, NATO aircraft taking part of Operation Deny Flight carried out an airstrike on the airbase of Udbina, Croatia, then part of the self-proclaimed Serbian Republic of Krajina. Krajina Serbs had been launching air attacks on neighbour Bosnia and Herzegovina from the base in support of allied Serbian forces there, specially during the battle of Bihac. NATO forces intervened in order to deter further attacks. Two anti-aircraft SA-2 missile sites that the Serbs had used to attack Bihac in the ground-to-ground mode and to engage NATO aircraft were also destroyed in the following days. The bombing of Udbina was the largest air combat operation in Europe since World War II, and the largest combat operation in NATO's history up to that time.