WHETHER the May 1 Croat military offensive is just another skirmish or the beginning of a new Balkan war is unclear. Serbs have launched rockets into Zagreb, something the Croats have long feared. All sides have been fighting in and around Bosnia since the cease-fire there ended. Bosnian Serbs have again shelled historic Dubrovnik, albeit with few press reports.
We suspect the Croat offensive is not designed to last long -- though it will be a huge boost and a significant experiment for Croatian President Franjo Tudjman. The UN peacekeepers and the Vance Plan cease-fire lines they are ''guarding'' are unpopular in Croatia, to say the least. Mr. Tudjman did not get much out of his recent bargaining with the UN; after 30 months of Serb occupation and a UN plan that has not delivered on any promises, Tudjman wants to show he can't be bossed around at home by foreigners.
The area under attack by the Croats is a small pocket in the upper Croat plains of Slavonia; it is almost cut off from Serb resupply. Croat militia there have long been aching to fight. This would probably be one of the first territories returned in any final settlement. Tens of thousands of Croat refugees from the nearby region of Osijek can still see their farms across a small river; they complain bitterly and pressure Tudjman.
Obviously, a wider war in Croatia would be inconvenient for the Western powers, who have settled into a relatively comfortable status quo on the Serb occupation of Croatia. Still, if the war in Bosnia showed anything, it showed that the so-called Yugoslav Army attack on Croatia in 1991 was the result of Serb aggression planned in Belgrade as part of Serbian President Milosevic's ''Greater Serbia'' ambition. Croats share some blame. But Serb-led attacks pulverized and flattened countless Croat homes and towns while Serbia proper, as in the later Bosnian-Serb war, was untouched. If nothing else, the Croat offensive reminds the world that the land is Croatia's, and that this UN member state someday wants it back.
Deny it or not, the Balkans are a bellwether for Europe and NATO. The brutality there is sadly defining the era. Experts who want to ''wash their hands'' of the Balkans and call this a civil war -- all evidence to the contrary -- do so in the face of history.