Mikhail S. Gorbachev, whose rise to power in the Soviet Union set in motion a series of revolutionary changes that transformed the map of Europe and ended the Cold War that had threatened the world with nuclear annihilation, has died in Moscow. He was 91.

Few leaders in the 20th century, indeed in any century, have had such a profound effect on their time. In little more than six tumultuous years, Gorbachev lifted the Iron Curtain, decisively altering the political climate of the world.