If you want to know when a war is coming, you just watch the United States and see when they start cutting down on their defense. It’s the surest barometer in the world.

Will Rogers (1879-1935) American humorist

Column (1933-05-16), “Daily Telegram”
    (Source)

Written while in Beverly Hills. Reprinted in his Autobiography, Donald Day ed. (1949), in a slightly different form:

If you want to know when a war is coming, just watch the U.S. and see when they start cutting down on their defense. It's the surest barometer in the world.

Another variant can be found in Bryan B. Sterling, ed., The Best of Will Rogers (1990):

If you want to know when a war might be coming, you just watch the U.S. and see when it starts cutting down on its defenses. It's the surest barometer in the world.