LONDON, JULY 5 -- The Western allies agreed today to issue a historic invitation to Soviet President Mikhail Gorbachev to attend a future meeting of the North Atlantic Treaty Organization, a decision that came as the NATO summit here opened a debate over how far and how fast the West should go in responding to a diminishing threat from Moscow.

The 16 Western alliance leaders gathered here agreed easily on a series of gestures aimed at reassuring the beleaguered Soviet leader and his critics that the era of Cold War confrontation has ended for NATO. But debate was sharper on the gut issues of changing the alliance's nuclear strategy and deployment of its forces.

The day was marked by discussion of communications with Moscow, including a letter from Gorbachev requesting Western aid and a proposal to send a high-level emissary to brief Gorbachev when the summit ends.

Working from a draft statement circulated last week by the United States, the group had little trouble endorsing a number of initiatives that encompassed what White House press secretary Marlin Fitzwater called "reaching out to old adversaries."

But a proposal by Bush to reword NATO's basic defense doctrine to designate nuclear arms as weapons of "last resort" initially ran into strong objections from British Prime Minister Margaret Thatcher.

The British dropped their objections to the phrase when NATO ministers endorsed other language making clear that the fundamentals of NATO's deterrence policy would not change. The group also struggled with proposals on how German troop strength, a key Soviet concern, should be addressed.

When foreign ministers concluded a late-night negotiating session, a senior U.S. official said "substantively, we got everything we wanted. We think some minor additions have made everyone happy."

West German officials said they were prepared to limit a unified Germany's future army to 390,000 troops -- the combined strength of U.S. and Soviet forces in central Europe under a pending agreement. The NATO leaders were expected to endorse a more general statement, proposed by West German Chancellor Helmut Kohl, that German troop levels should be negotiated in Vienna as part of an overall pact on reductions in conventional forces.

Among other proposals aimed at reaching out to the Soviets were the NATO invitation to Gorbachev, a proposal that East European countries base permanent liaison representatives at NATO headquarters and a package of proposals to establish a stronger and more permanent role for the 35-nation Conference on Security and Cooperation in Europe.

The group also discussed sending a high-level delegation to Moscow to underscore to the Soviets -- in the midst of a Communist Party congress there -- that they have nothing to fear from the new NATO.

Fitzwater described the discussions as generally harmonious and the allies as receptive to U.S. proposals. "But on the other hand," he said, "it's not business as usual at this meeting. We are at an historic turning point in terms of the future of the alliance."

The idea of Gorbachev's attending a session of NATO, Fitzwater said, is aimed at giving the Soviets "a special view of NATO that could help change their perspective on our years of adversarial relations."

Since its inception in 1949, NATO's role has been to defend Western Europe against attack by the Soviets and, after 1955, by the Warsaw Pact. Until Soviet Foreign Minister Eduard Shevardnadze visited NATO earlier this year, no senior Soviet official had been inside the defense center.

Bush, in his address to the other 15 leaders, promoted the Conference on Security and Cooperation in Europe as a forum for promoting free and fair elections, upholding the rule of law and protecting economic liberty.

In his draft proposal for consideration here, Bush had urged that the CSCE have a permanent staff located in Prague. A separate office in Berlin or Warsaw would serve as a conflict-prevention center, and a third, in Budapest, would monitor elections.

The United States wants to strengthen the CSCE, but in ways that do not impinge on the powers and influence of NATO. The Soviets have promoted the CSCE as a pan-European security structure that would eventually replace both NATO and the Warsaw Pact.

Officials said tonight that the issue of how Gorbachev will be briefed on the results of this summit remains to be settled. "Everything from A to Z is under consideration," said one official.

Foreign ministers worked late to draft a final communique, for release after Friday's closing session, that is likely to include several previously disclosed proposals. Among them are a nonaggression declaration between NATO and individual member nations of the Warsaw Pact and a call for removal of NATO nuclear artillery shells from Europe after certain conditions are met.

Fitzwater quoted Bush as telling the leaders: "We will show that NATO has a new dimension of cooperation with the Soviet Union and with the new democracies of Eastern Europe."

Although Fitzwater said the allies had been "generally positive" in response to Bush's proposals, administration sources said Thatcher reiterated objections that she had made to Bush in a weekend letter questioning any alteration in the basic NATO defense strategy of flexible response. Under that doctrine, the alliance could and would respond with nuclear weapons if the Soviets used their superior conventional military strength to launch an attack.

Bush, without calling for abandonment of flexible response, had proposed "a new definition that would truly make our nuclear forces into weapons of last resort." Thatcher objected that such language would eliminate the "ambiguity" that made the strategy an effective deterrent.

The British leader argued that piecemeal changes should not be made in the NATO nuclear doctrine until a full review of its purpose and needs, now in progress, is completed.

"The starting point must be the strategy and forces we need for the future, not what we can discard," Thatcher told the other leaders gathered in Lancaster House, a 19th-century mansion in central London.

Tonight, however, British officials said they had accepted language in the draft communique that used the words "last resort" but sufficiently qualified the change to provide allied forces the flexibility to use nuclear weapons when they deemed it necessary.

Thatcher's speech at this morning's opening session was an impassioned plea for caution. She recited a list of the weaponry she said the Soviets were still producing -- four strategic missiles and 100 tactical air-to-surface missiles per week, six tanks and two combat aircraft each day and a submarine every six weeks -- and warned that "Soviet military might remains formidable."

Thatcher's aides said she had received a letter from Gorbachev on the eve of the summit in which he made his most direct appeal for Western economic aid. He told Thatcher that the Soviets need both short- and long-term "practical support" for his program of economic and political reform to succeed, aides said.

Gorbachev pledged in the letter that the Soviet Union would cooperate with a study of its needs authorized last week by the leaders of the European Community and said he hoped the study would produce "positive conclusions."

The letter also expressed Gorbachev's hope that the NATO summit would make a "constructive contribution" to the development of European security.

Aid to the Soviets was not on the formal agenda at today's session, but a senior aide to Kohl said he was certain the West German leader would raise the issue informally.

The West Germans have pressed for a Western commitment to aid as part of their attempt to win Soviet acquiescence to a united Germany in NATO. Such aid "was not a private hobby of Chancellor Kohl's," said Hans Klein, a minister of state in Kohl's office. "It lies in the common interest of all free democracies."

London correspondent Glenn Frankel contributed to this report.