American ambivalence about European engagements is almost a cliche, despite the US role in two world wars, its Cold War leadership and its very risky nuclear guarantee in NATO. It is nonetheless an historical reality. Throughout the twentieth century, a genuine - if limited - isolationist impulse was driven mainly by political and cultural currents that ranged from ambivalence to open hostility towards alliances with the major European powers. Advocates of a strong US engagement usually carried the day, but not before a debate that injected some damaging uncertainty into the alliance system. Moreover, this vigorous Atlanticism did not always prevail: its emblematic failure was of President Woodrow Wilson's attempt to secure American participation in the League of Nations.