President Reagan, trying to soothe European leaders before a North Atlantic Treaty Organization meeting this week, pledged yesterday that he would "never sacrifice the interests of our allies and friends" for any arms control agreement with the Soviet Union.

Reagan is scheduled to leave this morning for a three-day trip to Brussels, where he is to meet with NATO leaders in an attempt to lay groundwork for allied unity in advance of an anticipated Moscow summit, probably in late May, with Soviet leader Mikhail Gorbachev.

Administration officials acknowledged privately yesterday intense concern among the allies about the potential impact of a treaty signed by the two leaders here that would eliminate the superpowers' medium-range and shorter-range nuclear missiles and about continuing U.S.-Soviet negotiations on an accord that would make deep cuts in strategic nuclear arsenals.

"Let's face it," one senior official said. "Ever since Reykjavik" when Reagan and Gorbachev discussed abolishing nuclear weapons, "there has been concern in Europe that we're going to fold up the U.S. nuclear umbrella. That isn't going to happen, and the president wants to say so directly."

Reagan's forum was the national convention of the American Legion, where he had provoked controversy during the 1980 election campaign by terming the Vietnam war "a noble cause," a phrase he repeated yesterday.

But aides said the speech was intended to send a message to Europe that Reagan's anticommunism and concern for the alliance remains undiminished despite his efforts to reach arms accords with Gorbachev.

"We have let there be no doubt in the Soviets' minds -- an attack on free Europe would be the same as an assault on the United States," Reagan said. "The core of our foreign policy and of our national security is our permanent partnership with our fellow democracies in the Atlantic Alliance on which the cause of freedom so critically depends."

He tried to allay European concerns that reductions in superpower nuclear arsenals would give the Soviets a decisive advantage in conventional military forces.

He said that, while pushing for reduction in strategic nuclear weapons and a global ban on chemical weapons, "the serious imbalance of conventional forces in Europe" must have an equally high priority.

"This imbalance represents an unacceptable threat to the West," Reagan said. "Warsaw Pact tanks and artillery far outnumber our own. Ours are positioned for defense, theirs for an offensive attack."

While pledging continued U.S. support for NATO, the president also gently encouraged the allies to do more in their defense, saying the European nations had risen from the ruins of World War II to prosperity propelled by American aid.

"And so, I would submit that now the alliance should become more and more one among equals, an alliance between continents," he said.

Reagan also renewed his appeal for U.S. aid to the Nicaraguan contras, an issue likely to come up for another vote in the House while he is in Brussels.

The president said that "unless the freedom fighters remain a viable force," the Central American peace process "will quickly become what it was before -- an empty charade, dragging endlessly and fruitlessly on, while the Soviets continue their military buildup on the American mainland."

He also promised that he would not "agree to any steps that would put the Afghan freedom fighters or Afghan hopes for self-determination at risk."

In an interview with The Washington Post last week, Reagan said he believes Gorbachev's declaration that he intends to withdraw Soviet military forces from Afghanistan. Reagan said yesterday that the Soviet decision represented the wisdom of the policy of "peace through strength" that he promised to pursue when campaigning for the presidency eight years ago.

Reagan is scheduled to leave the White House early this morning after a departure statement on the south grounds. He is to arrive at Zaventem Airport in Brussels at 9:45 p.m. local time and spend the night at Chateau Stuyvenberg, a government guest house.