MRS Margaret Thatcher's criticism of the West for behaving ''like an accomplice to a massacre'' for its failure to arm Bosnia's Muslims may sound glib and emotive. However, it is as much an attempt by the former prime minister to undermine her successor as a contribution to the debate on curbing Serb excesses. Her call for the Serbs to be given an ultimatum to accept the Owen-Vance peace plan or face Western military intervention on the Muslim side is far removed from the peacekeeping role to which Britishand United Nations' troops are currently assigned. However, Mrs Thatcher's tirade raises valid questions about the value of the international community's attempts to remain even-handed in a dispute where the two sides are so unevenly matched. To shorten the odds, the United States favours partially lifting the arms embargo now covering all sides, and letting the Muslims re-arm against the better-equipped Serbs. Washington believes failure to arm the Muslims now will ensure the murder, the rape and the ethnic cleansing continue unabated. The case against lifting the embargo has been eloquently made by the British Foreign Secretary, Mr Douglas Hurd, who warned it would create a ''level killing field'' and prolong the war. It would also ensure the suffering was more equally distributed on both sides. This newspaper has argued the Serbs should be held to a revised version of the Vance plan by the presence of international troops. Arming their opponents would achieve the opposite result. The current peacekeeping operation has neither the means nor the mandate to be effective. There is a world of difference between a well-armed peacekeeping operation equipped to use force against either side, and a partisan army allied to the Muslims against the Serbs. The international community's role should not be to tip the scales in favour of one side, but to impose and enforce a just peace.