Leading Article: At last, the UN draws a line

WHAT did the Bosnian Serbs expect? Their bombing attack on a munitions factory in Muslim-held central Bosnia was not just a flagrant breach of the no- fly zone agreed by the United Nations, but - as the first such act of war since Nato warplanes began to enforce the UN resolution last April - a significant escalation of hostilities. As US Admiral Mike Boorda observed drily yesterday: 'If it was a test (of UN resolve), I think we passed the quiz.' Nato's first hostile action since it was founded in 1949 had taken place 'out of area'.

Bosnian Serb motives for the assault are likely to remain obscure. Their policy has, until their recent implementation of the Sarajevo ceasefire and withdrawal of much of the artillery that has been pounding the Bosnian capital, been based on an assumption that UN/

Nato threats of air strikes would not be implemented. There is a contradiction between the Bosnian Serbs' apparent awareness of a new toughness in Western policy over Sarajevo and the seemingly foolhardy nature of this attack.

Perhaps they felt emboldened by the increased involvement - and apparent support - of their Russian fellow Slavs in and around Sarajevo. They should, however, have realised that the Russians also approved the relevant UN resolution, and could therefore not rationally object to its implementation following so gross a violation.

The main factor behind the Serb strike was probably fear of the effects of the Croat-Muslim deal the US has been attempting to broker. Latterly, more than half the fighting in Bosnia has been between Bosnian Muslims and Bosnian Croats backed by Croatia proper. The Americans have suggested the two sides form an alliance that would balance the strength of the Bosnian Serbs. A ceasefire was agreed last Thursday, but has not yet been convincingly established.

The Serbs doubtless fear that when it is in effect, the Muslims will turn their increasingly effective ground troops against them. Hence their pre-emptive strike against one of the Muslims' most substantial munitions factories. Had the bombardment taken place without any punitive action, the credibility of both the UN and Nato would have received another serious blow. As it is, the effectiveness of the Nato action is a setback to Serb self-esteem as well as a significant loss of air force hardware.

Across Europe and in the US, yesterday's action will come as welcome evidence that the democratic West is capable of implementing its promises. For far too long the Serbs have pushed against the boundaries of its tolerance and found them elastic. Now at last a line has been drawn. The general reaction is likely to be a relieved: 'At last]'.