Bosnian Serbs said they would cut off commercial and civilian traffic going in and out of Sarajevo tomorrow-- returning the city to its crippled, besieged state of a few months back. The Serbs claimed they had to impose new strictures to stop Muslims from smuggling arms into Sarajevo. To insiders, however, it looks more like a test of the West's resolve, following the Serbs' rejection of a peace plan backed by the U.S., Russia and much of Western Europe. "The Serbs know that if the West doesn't do anything about Sarajevo, it won't do anything to force them to accept the peace plan," says TIME Central European bureau chief James Graff.RUSSIA . . . PLAYING THE BOSNIA CARD? It's a sideline squabble to the Bosnian conflict, but it may have significant consequences: Russia is insisting that peacekeeping operations in the embattled former Yugoslavia remain in the hands of the United Nations--not NATO. But with the U.N. threatening to pull its troops out, the North Atlantic Treaty Organization remains the only legitimate body left to enforce peace in Bosnia. So why are Russians choosing this issue--and time-- to get difficult? Quick entry into NATO is their goal, says Graff. "I can see this becoming a catalyst to forming some kind of early alliance between NATO and Russia."