Ever since Franklin, Jay, and John Adams gained the Mississippi as the western boundary of the newborn United States in 1782, Americans have talked republican ideals but behaved imperially. In a string of episodes, the nation soon pushed all the way to the Pacific. First came acquisition; then displacement of the previous occupants and American settlement. Continental empire-building created a national habit. A second, offshore empire followed and since 1945, a global one. Why are we imperial? Here is the history.