The editorial ''End the Bosnia Posturing,'' June 9, rightly urges international action against ''a one-sided genocidal aggression that threatens the long-term security of the West.'' Although a solution to that catastrophe poses an extreme challenge to the imagination, there are at least two things the United States and its allies should do to help catalyze a determined effort by all sides to reach a settlement. One step is through military action, the other is through diplomacy:
r The military measure. To the extent that military aid continues to flow from Serbia to the Serb forces in Bosnia, NATO air forces should initiate - and continue for as long as necessary - the bombing of every identifiable bridge and road on that border. Such a response cannot stop electronic assistance (via radio or computer), but NATO's message would be powerful nonetheless.
r The diplomatic measure. The NATO powers should strongly declare their determination to deny United Nations membership to a Greater Serbia formed outside the framework of a negotiated settlement of the Balkan crisis. Trade and economic contacts would also be denied.
David J. Steinberg Alexandria, Va.
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