DEFYING EXPECTATIONS, opponents of Serbian strongman Slobodan Milosevic turned out in massive numbers for a demonstration last week. The 100,000-plus protest was but one step on an arduous road toward regime change; regime change in turn is only one step on an even more arduous path of reflection and acknowledgment that Serbia must follow after a decade of nationalist war-making. But, in both cases, first steps are not to be sneezed at.

Mr. Milosevic enjoys a presumption of invulnerability. But he is not invulnerable, not anymore. Many Serbs now understand he has brought them nothing but misery. They now live in Europe's poorest country, shunned by the rest of the continent. They know they will remain isolated as long as Mr. Milosevic, indicted for masterminding Serbia's brutal campaign against civilian Kosovars, remains in power. The apt question now is whether Mr. Milosevic's exit will be peaceful or bloody.

Much is made of disunity within the opposition, but it is neither surprising nor fatal. One can hope for democracy or unanimity, but not both. The success of last week's demonstration, and of the provincial protests that preceded it, reflects increased cooperation among opposition parties and between those parties and the church.

A greater worry is the nationalism of much of the opposition. To be opposed to Mr. Milosevic in Serbia today is not the same as to acknowledge the criminality of his wars against Croatia, Bosnia and Kosovo. Some opponents want to face up to Serbia's guilt; some share in it; some will adopt any position that will bring them to power. It is important, therefore, for the West to remain united, not only in pushing for Mr. Milosevic's ouster and arrest, but for democratization, cooperation with the International War Crimes Tribunal, release of Kosovar prisoners and support for human rights.

Still, Mr. Milosevic's ouster is a crucial beginning, one that would allow the West to begin to engage Serbia instead of isolating it. Unions that assisted Solidarity in Poland should be playing a similar role in Belgrade. American church groups that were so eager to meet with Mr. Milosevic during the war should be engaging with the Serbian Orthodox Church now that it has moved into opposition. The West should be assisting Serbia's independent media, and NATO should move aggressively to arrest Serbians indicted for their part in the Bosnian war. Serbians will determine their own future, but they could use help.