"We were sure that we would win, that we should score the first great triumph in a mighty world-movement."--Theodore Roosevelt, 1904

Americans like to think they have no imperial past. In fact, the United States became an imperial nation within five short years a century ago (1898-1903), exploding onto the international scene with the conquest of Cuba, Puerto Rico, the Philippines, Hawaii, Guam, Samoa, and (indirectly) Panama. How did the nation become a player in world politics so suddenly-and what inspired the move toward imperialism in the first place?

The renowned...