WHEN REAGAN MET LENIN: Three decades ago an American president issued a cry for freedom at Moscow State University.

Twenty eighteen has been full of backward glances. The most frequent subject has been that singular year 1968, fulcrum and focal point of everything we sum up in that dread phrase “the ’60s.”

A less depressing prospect is on view if we travel back not 50 but 30 years, to May 31, 1988, when Ronald Reagan, in the last year of his presidency, delivered one of his most magnificent speeches. At the end of his first inaugural address, Lincoln famously spoke of the “mystic chords of memory” that, beckoning toward truths that transcend party differences, recall us to the “better angels of our nature.” Reagan did something similar in his speech before a packed auditorium of students at Moscow State University.

It was the last day of his fourth and final summit with Mikhail Gorbachev. They had first met on neutral ground, in Geneva, in 1985, and the next year in Reykjavik, Iceland. A Washington summit followed in 1987. Now Reagan had traveled to the Soviet capital. The ostensible purpose of all these meetings was to work out arms-control agreements, and the two had made significant progress. In Washington, they had signed a pact to eliminate a whole class of intermediate-range nuclear missiles. They had also laid the groundwork for the future reductions that would come with the 1991 Strategic Arms Reduction Treaty.

But Reagan never regarded his meetings with Mr. Gorbachev as pertaining solely to arms control. Arms control was merely the pretext for a more fundamental challenge. This is the deep point of Bret Baier’s forceful new book, “Three Days in Moscow: Ronald Reagan and the Fall of the Soviet Empire.” Mr. Baier traces the arduous evolution of Reagan’s diplomatic efforts with the Soviet Union, from before his famous “evil empire” speech in 1983 through that final summit in Moscow and beyond. If the theme is diplomacy, the underlying purpose is liberty.

In 1977, noting to a friend that “a lot of very complex things are very simple if you think them through,” Reagan crisply summed up his theory of the Cold War: “We win, they lose.” He never lost sight of that conviction. Nor did he waver in his understanding that weakness is an invitation to conflict. He did, however, understand that victory would belong in the end not to one nation over another, but to one political-moral idea over another. Freedom must triumph over totalitarianism.