The Reagan administration warned Congress yesterday that legislation limiting U.S. troop strength in Europe would "signal a broad U.S. retreat from its responsibilities" just when the new Soviet leadership is assessing Western cohesion.

Undersecretary of State Lawrence S. Eagleburger also told members of the Senate Foreign Relations Committee that measures in the defense appropriations bill currently being considered by Congress "would, among other things, . . . interfere with our ability to meet our commitment to modernize NATO's nuclear forces" in Europe and undercut the U.S. negotiating position at the nuclear arms talks in Geneva.

The bill contains provisions that would reduce by 23,900 the planned U.S. troop strength in Europe and eliminate American funding for 93,000 West German reservists to support the U.S. troops there under the Host Nation Support program. It also would reduce funding for ground-launched cruise missiles and for stockpiling equipment for U.S. forces to be deployed rapidly overseas.

These provisions, which received overwhelming support in Senate committee action, reflect a growing sentiment within Congress that European nations must bear more of the burden for Western defense while the U.S. expands its military role elsewhere in the world, often to Europe's benefit.

"Passage of this legislation would be a fundamental departure from the historical bipartisan post-war U.S. approach to national security," Eagleburger argued. "Never has the American role in the defense of Europe been reduced through legislation. Never has the U.S. backed away from its NATO commitments. And never have the elected representatives of the American people voted not to stand by our allies and back up our defense commitments.

"Are we really ready now to take such a fateful step? Do we really want to greet the new Soviet leadership with a sharp deviation from the policies that have so successfully preserved Western security and American leadership in Europe?"

In 1971, an attempt led by Senate Majority Leader Mike Mansfield to reduce the U.S. force in Europe to 250,000 was rejected by the Senate by a vote of 54 to 39. Two months ago, the Senate Appropriations defense subcommittee voted to freeze American troop strength in Europe at the 1980 level of 331,700, compared to the 355,600 proposed by the administration.

"We've been increasing our troop strength to make up for them decreasing theirs. I don't believe in this constant escalation of troop strength," argued the subcommittee's chairman, Sen. Ted Stevens (R-Alaska).

Senate sources indicated yesterday that even if a somewhat similar House bill does not reach the Senate in time for final action during the three-week lame-duck session of Congress, there is likely to be an attempt to put some kind of peacetime ceiling on U.S. troops in Europe into a continuing spending resolution that would have to passed.

"Proponents of this legislation may claim that our doing less would jolt our allies into doing more," Eagleburger said. "I see no basis for such wishful thinking. U.S. cuts would have the opposite effect. If we do less, the Europeans will do less, and we will be less secure."