The United States has often played a pivotal role in my political life, beginning 50 years ago when I was a student of international relations at a Moscow university.

At that time, Soviet propaganda was well-practiced at denouncing Richard Nixon for rejecting the Kremlin’s dogma that in politics, the ends justify the means. Mr. Nixon had argued during his 1960 presidential campaign that the American democratic system recognizes a standard of moral truth that allows the individual to say to government, “Thus far may you go, but no farther.” If what Mr. Nixon said was true, many of us in the Soviet Union thought, then America is on the right side of history.

The Kremlin would later exploit the Watergate scandal to sneer at Mr. Nixon’s — and by extension, America’s — allegiance to moral truth as nothing more than hypocrisy. But what looked like an easy propaganda victory turned out to be Pyrrhic. When Republicans joined Mr. Nixon’s Democratic opposition in Congress and forced him to choose between resignation or impeachment, the Soviet dissident newsletters retorted that the standard of moral truth had proved to be real in America after all. And they pointed out that the investigation of the president of the United States was initiated by two young reporters, representatives of a free press. The Kremlin found no counterarguments, except to condemn the press as enemies of the people and dissidents as traitors.

The Iron Curtain was not enough to block the words of moral truth from Washington. Gradually more and more people in Russia heard and heeded them. The words spoken by successive presidents, members of Congress and many ordinary citizens were loud and clear — and when buttressed by genuine efforts to live up to them, they were the most powerful weapons in the Cold War. Unlike nuclear missiles, free speech, and the moral truth it brought, was not an element of the superpower struggle to be found in equal measure among both rivals. It was a unique and essential advantage on the American side.