WASHINGTON — These days it is hard to imagine a single presidential speech changing history.

But two speeches, given back to back by President John F. Kennedy 50 years ago this week, are now viewed as critical turning points on the transcendent issues of the last century.

The speeches, which came on consecutive days, took political risks. They sought to shift the nation’s thinking on the “inevitability” of war with the Soviet Union and to make urgent the “moral crisis” of civil rights. Beyond their considerable impact on American minds, these two speeches had something in common that oratory now often misses. They both led quickly and directly to important changes.

On Monday, June 10, 1963, Kennedy announced new talks to try to curb nuclear tests, signaling a decrease in tension between the United States and the Soviet Union. Speaking at American University’s morning commencement, he urged new approaches to the cold war, saying,  “And if we cannot end now our differences, at least we can help make the world safe for diversity.”...