Months of policy errors undermining U.S. influence in Europe lie behind those summit-eve State Department leaks on new ''assurances'' for Mikhail Gorbachev about Germany.

Only hours before President Gorbachev stepped onto U.S. soil did President Bush and Secretary of State James A. Baker III correctly read what the signs long had been telling them. Moscow will not accept military membership in NATO by a unified Germany, as the United States has always assumed. What's more, the Kremlin will insist on complete military reciprocity with the United States in any final German settlement.

No matter how hard the two presidents try to paper over this seminal disagreement on the world's biggest political question, Gorbachev's demand for full equality has chilled the Americans. Add this to Baker's startling proposal limiting American troops outside Germany and Belgium to 30,000, and the United States will be in a military straitjacket on the continent. This overhangs a summit unlikely to reach final settlements on strategic arms, trade or the Baltics.

The wonder here is that respected professionals on Bush's National Security Council staff and in Baker's State Department did not see Gorbachev's runaway locomotive, warning flags flying, come tearing down the tracks at them. Gorbachev's German-policy aide, Nikolai Portugalov, told Hamburg's authoritative Die Welt on May 21 that his boss will demand ''the quick mutual withdrawal of all foreign troops {that is, American and Soviet} from German soil.''

Portugalov is at the core of Soviet decision-making on Germany. He said the West must understand the possibility that Moscow's 380,000 troops ''will stay {in East Germany} on an equal basis as long as a foreign Western military presence continues'' in West Germany. And NATO? ''Membership in NATO of a united Germany is absolutely unacceptable.''

Gorbachev himself told Time he is considering a way ''to synchronize the political and disarmament processes with the pace of German unification.'' In other words, Soviet and American troops would march out of Germany, one-for-one, as the price of reunification.

U.S. officials, spurred on by West Germany, have been insisting that this is only a Gorbachev bargaining tactic: overstating a demand to achieve something less. Bonn also suggests the"German politicians, not the United States, will control the U.S. troop presence." Soviets might accept a limited German partnership in NATO, political but not military as with France. In conversation with U.S. officials, Gorbachev has mentioned the French model. But a non-military role for Germany would eviscerate basic U.S. policy in Europe.

That is why Baker's eye-popping proposal for a ceiling of 30,000 on American troops outside NATO's ''central sector'' (Germany and Belgium) is so worrisome. Baker dropped the promise at the alliance's Ottawa meeting in February after clearing it with Gorbachev in Moscow. But the number was never discussed with the Joint Chiefs of Staff, never brainstormed by Pentagon civilian strategists and never given to NATO members before Ottawa.

For three months, the 30,000 limit has been under quiet attack by congressional leaders in closed-door White House meetings. They worry that it will cripple U.S. power if American troops are sent packing from Germany.

''That was a serious error,'' Sen. Sam Nunn, chairman of the Senate Armed Services Committee and the Democratic Party's arbiter on military strategy, told us. Nunn believes it ''inevitable'' that U.S. forces will be out of Germany within 10 years. He says Baker's ceiling of 30,000 in all the rest of Europe will not be enough to sustain the U.S. Navy's obligations in the Mediterranean or U.S. air bases scattered throughout non-German Europe. They are needed because military resurgence by the Soviet Union cannot be ruled out.

When Sen. Wyche Fowler (D-Ga.) asked Baker at a Senate Budget Committee hearing what would happen to the United States in Europe if American troops had to leave Germany, Baker ''got upset,'' one senator told us, and shot back: ''That's not going to happen.'' But his prediction is undermined by West German polls showing that 70 percent-plus want Americans out and by Gorbachev's linking Soviet withdrawals to U.S. withdrawals.

German politicians, not the United States, will control the U.S. troop presence. Administration officials who insist that Chancellor Helmut Kohl will never accept Gorbachev's demand cannot know for sure. These officials were distraught when word leaked of a secret trip to Moscow last week by Kohl's national security adviser, Horst Teltschik. It suggested possible German-Soviet convergence.

Four days at the summit cannot come close to finding definitive answers to the lethal German question under best of circumstances. In the actual case, Bush will be fortunate to hold his own.