In the first of five Boston-area panel discussions about post Cold War military spending, three experts agreed last night that the United States must pursue an active role in world leadership.

The panelists, moderated by Joseph S. Nye, director of the Harvard Center for International Affairs (CFIA), swapped opinions on the topic "Reassessing Our Military Priorities: What are the new threats and what should the U.S. role be?"

The three speakers all emphasized the need for U.S. cooperation with other countries to maintain international security.

"We are a world leader. We can't retreat to fortress America," said U.S. Army Col. Keirn C. Brown, a fellow at CFIA.

He was joined on the panel by Randall Forsberg, executive director of the Institute for Defense and Disarmament Studies, a non-profit organization in Cambridge, and Stephen Van Evera, assistant professor of political science at MIT.

Van Evera and Forsberg said that future U.S. foreign policy should focus on Europe.

Van Evera said that "in western Europe, war is almost unthinkable," but he was more pessimistic about the chance for peace in the former Soviet Union and Eastern Europe.

"History warns us that the collapse of great empires has been the cause of great wars," he said.

Van Evera said the U.S. should take advantage of its monetary aid programs to Eastern Block countries to influence their policies.

And Forsberg said that right now is a crucial time to redefine the relationship among states from the present "anarchic hegemonic" one to a more peaceful, non-violent international code.

On the topic of military spending, Brown was the only panelist who did not favor a drastic military budget cut.

Van Evera, on the other hand, said that a 35% reduction from the current spending level would be reasonable.

Forsberg agreed that "very substantial reductions" are necessary, since the end of the Cold War has eliminated the need for a whopping defense budget.

Presently, no developing nation poses a threat great enough to warrant massive spending, she said.

Last night's program, held in the ARCO Forum at the Kennedy School of Government, kicks off the two-week series of panel discussions. Students at Tufts, MIT, Boston University and U. Mass.-Boston have organized the remaining events to follow at their respective schools.