World War I broke out largely because of an arms race, and World War II because of the lack of an arms race.

World War I broke out largely because of an arms race, and World War II because of the lack of an arms race.

第一次世界大战的爆发主要是因为一场军备竞赛，而第二次世界大战则是因为没有军备竞赛。

-- Herman Kahn 卡恩