Much of the Serb-held land in Croatia, the Krajina region, has now been lost in a spectacular but foreseeable defeat.

If, on the other hand, Bosnian forces hold on to the Bihac pocket, Serb-held land in western Bosnia and Croatia will remain vulnerable.

Brcko dominates a corridor linking Serb-held land in western Bosnia and in Croatia with Serbia proper to the east.

But the Bosnian Serbs have made it clear that "the map," the proposal to reduce Serb-held lands to 49 percent, is utterly unacceptable.

The final exchange would be Serb-held land to the northwest of Sarajevo that would link the Bosnian capital with the Government-held heartland around Zenica.

They would trade Muslim-held land in eastern Bosnia, which the plan leaves with the Muslims, for Serb-held land around Sarajevo.

The United States and the major European powers urged Croatia today to halt its offensive to recapture Serb-held lands, but Washington's criticisms were muted.

This struggle in Serb-held lands is likely to unfold over the next several years.

Those corridors would come from Serb-held land.

They have already agreed to form three autonomous ethnic republics, but the Muslims insist on more Serb-held land and an access to the sea through Croat territory.