President Reagan yesterday pledged that U.S.-Soviet arms-control agreements will not diminish the longstanding U.S. commitment to defend Western Europe with U.S. troops and nuclear weapons.

Reagan said that the Atlantic alliance formed after World War II remains "the core of America's foreign policy and of America's own security," despite a new U.S.-Soviet treaty that would scrap the superpowers' land-based missiles with a range of 300 to 3,100 miles.

"If you are not at peace, we cannot be at peace," the president said in a speech televised to European audiences by satellite on the U.S. Information Agency's world network. "An attack on you is an attack on us. . . . Simply put: An attack on Munich is the same as an attack on Chicago."

The speech was intended to reassure Europeans in advance of next week's North Atlantic Treaty Organization meeting in Brussels, where Reagan will confer with European leaders and foreign ministers. A senior U.S. official said the meeting is expected to "demonstrate allied unity" in advance of the Moscow summit that Reagan and Soviet leader Mikhail Gorbachev have agreed to hold this spring.

In his remarks yesterday, Reagan said that the new Intermediate-Range Nuclear Forces (INF) Treaty would not end the U.S. nuclear presence in Europe. He said approximately 4,000 nuclear missiles and 300,000 U.S. troops would remain in Europe and added that the United States would keep its forces "strong and up to date."

"Our goal is not a nuclear-free, or a tank-free, or an army-free Europe," Reagan said in the speech, which was televised from the White House's Map Room, which President Franklin D. Roosevelt used for strategy sessions during World War II. "A war-free Europe is what we have today. A war-free Europe is what we want to preserve," he said.

Reagan promised to press ahead with attempts to reach an agreement with the Soviets that would halve the strategic, or long-range, nuclear arsenals of the superpowers. But he also said he would "press forward with our Strategic Defense Initiative," the missile-defense plan that the Soviets have said is an obstacle to a strategic agreement.

"It would be a fatal mistake not to pursue this program," Reagan said. "Even before it becomes 'leak-proof,' strategic defense will strengthen deterrence. It can make anyone who might think about disabling the West with a first strike think again because it will undermine Soviet confidence in the military success of an attack."

Reagan said his administration did not have illusions about Soviet motives as it pursued agreements on nuclear and conventional arms and on chemical weapons. He said that the policy of glasnost, or openness, initiated by Gorbachev "coexists today with the reality of political repression in the Soviet Union."

"We cannot afford to forget that we are dealing with a political system, a political culture and a political history going back many decades, even centuries," Reagan said. "Swings between glasnost and the gulag are not new or even peculiar to the Soviet regime. In history they recurred again and again as the throne passed from czar to czar and even within the reign of a single czar. We cannot afford to mortgage our security to the assessed motives of particular individuals or to the novel approaches of a new leadership, even if we wish them well," he added.