In this light it is hard not to see the Croatian victory as welcome. Last month, the Serbs looked unassailable, overwhelming the United Nations "safe areas" of Zepa and Srebrenica, slaughtering hundreds, perhaps thousands, of people put to flight by their advance. Now, suddenly, they no longer seem invincible. Dreams of a Greater Serbia, embracing large parts of Croatia as well as Bosnia, have been shattered.
But as ever in the Balkans justice is very rough. The conflict in former Yugoslavia is again displacing, possibly forever, peoples who have lived in an area for generations. The Krajina Serbs who are escaping the Croatian offensive have occupied the region for 300 years. Their culture now faces extinction. As they flood into Bosnia, another humanitarian disaster, involving up to 150,000 people, is in the making.
The challenge now for the Croats is to distinguish themselves from the Bosnian Serbs by the manner of their victory. They must keep their troops disciplined and allow the UN and the Red Cross into the conquered lands. In the longer term the Krajina Serbs should be given autonomy.
If the far-sightedness of the Croats' political strategy fails to match its military might, there is a danger that this latest offensive will escalate into a wider war. Serbia itself has so far abandoned the Krajina Serbs to the Croatian attack, so limiting the conflict. But if the Croats, overconfident in victory, advance too far or massacre Krajina Serb civilians, Serbia will find it hard to stand back.
To avoid the war widening, Croatia's most influential friends, the United States and Germany, must impress upon the Zagreb government that its current offensive should not be extended into eastern Slavonia.
Equally, the West must not sanction a deal between Serbia and Croatia to carve up Bosnia. The attractions of such a deal are that it might bring more order to the region. But the cost would be paid by the Bosnian Muslims, the very people who enjoy most sympathy in the West. The fear is that the Croats may decide, now they are militarily strong, that the time is right to abandon the Muslims as allies and strike a deal with Serbia proper which would give the Bosnian Serbs a freer hand in Bosnia.
The Bosnian Serbs, inflamed by their defeat this week, their ranks strengthened by refugees from Krajina, might seek to extend their ethnically pure homeland at the expense of the Bosnian Muslims. So the UN and Nato could soon face a Serbian challenge to the four remaining UN "safe areas", which the international community is pledged to protect.
The UN must be genuinely prepared to use whatever air strikes are necessary, to rebuff Serbian aggression against the Bosnian safe areas. It must also do as much as possible to protect the fleeing Krajina Serbs and bring pressure to bear on the Croats not to ditch the Muslims in favour of a deal with Belgrade.
The Serbs, chastened by their experience in Krajina, faced with a still solid Croat-Muslim alliance and a firm UN commitment to protect Bosnia, might at last be persuaded to negotiate a just settlement.