BRUSSELS, MARCH 1 -- President Reagan, vowing to strengthen the Atlantic Alliance, arrived here today for a NATO summit at which western leaders are expected to call for sharp reductions in Warsaw Pact conventional forces.

Reagan, who was greeted here by Belgian Prime Minister Wilfried Martens and U.S. Secretary of State George P. Shultz, had no arrival statement. But before leaving Washington this morning he stated the principal theme of his visit by declaring, "We will never sacrifice the interests of this partnership in any agreement with the Soviet Union."

U.S. and allied officials said that the NATO leaders would issue a declaration, possibly as early as Wednesday, spelling out alliance objectives for future negotiations on reducing the conventional forces of both sides in Europe. Reagan said in a speech in Washington yesterday that "the serious imbalance of conventional forces in Europe . . . represents an unacceptable threat to the West."

In his departure remarks the president repeatedly stressed the "unwavering commitment" of the United States to the western alliance.

"My Atlantic colleagues and I will rededicate ourselves to maintaining the deterrent that has protected our freedom and prosperity for almost 40 years," Reagan said. "I will repeat to my colleagues my strong conviction that American troops will remain in Europe, under any administration, as long as Europeans want them to stay."

"We can be rightfully proud of these 40 years of peace that our common commitment has brought," the president continued. " . . . We seek nothing less than permanent peace with freedom in Europe and the North Atlantic."

Although the first NATO summit in six years was designed to be a celebration of alliance unity, nagging disagreements over defense strategy were much in evidence on the eve of the two-day meeting.

Diplomats still were struggling here to find language to bridge differences for two joint NATO statements to be issued during the summit.

West Germany, with tacit support from the United States, was resisting British pressure for a strongly worded affirmation of the need to modernize European-based nuclear arms to maintain an adequate deterrent, NATO diplomats said.

The Bonn government fears that deployment of new, updated nuclear weapons in West Germany would arouse strong public protests.

Efforts to find a consensus were complicated by an apparent split within the French government over the issue, West German and British officials said. The topic was to be handled in a declaration on overall NATO policy to be issued Thursday, at the end of the summit.

Officials predicted that the dispute would be solved by calling for nuclear weapons to be kept up to date, but without making specific comitments or using the politically sensitive word "modernization."

The British are not in a position to force the issue because Reagan agreed with West German Chancellor Helmut Kohl in Washington two weeks ago that there was no immediate need to make modernization decisions, diplomats said.

A display of solidarity is particularly important now, at a time when relations between the United States and the Soviet Union are undergoing major changes, they said. It is important to reaffirm NATO's cohesion following the signing of the Intermediate-range Nuclear Forces (INF) Treaty in December and in advance of the U.S.-Soviet summit in Moscow later this year, they added.

"Every time the United States and the Soviet Union do business, it leads to a certain amount of nervousness and anxiety among the allies," a U.S official said.

One of the few surprises to emerge before the summit was the apparent division between French President Francois Mitterrand and Prime Minister Jacques Chirac over modernization of NATO's short-range nuclear weapons.

Mitterrand, aligning himself with the West Germans, has said in recent days that he opposes deciding now on modernizing such arms.

Chirac, a conservative who governs in an uneasy partnership with the Socialist Mitterrand, was said to be backing the British.

"I think Mr. Mitterrand's statement has caused some surprise, even to his own prime minister, because it is out of line with his own government's policy," British Foreign Secretary Geoffrey Howe said in a radio interview this morning.

Reagan was hoarse from what aides described as a slight cold when he spoke today on the South Lawn of the White House, but spokesman Marlin Fitzwater said the president was "feeling fine" now. "He said he looks forward to this meeting," Fitzwater said. " . . . He said he wants give the allies his sincere assurances and support for the alliance."

However, Reagan told reporters later that "my allergy is still bothering me," and then turned to Shultz and said, "My ears are still stopped up." The president then gave a throaty cough.