America's allies got the candidate they overwhelmingly preferred, but that doesn't necessarily mean America's relationships with the globe will warm instantly. In fact, history is strewn with rock-star presidents whose first years were terrific disappointments for U.S. allies.

John F. Kennedy, the son of an ambassador, "considered himself a citizen of the world" and thought he could quickly assert "America's position," says David M. Kennedy, the Stanford historian. Yet it took him barely three months to make his first major mistake: the disastrous Bay of Pigs invasion of Cuba. JFK biographer Robert Dallek says many European allies were horrified by Kennedy's rashness and were soon appalled anew when Kennedy, ignoring a direct warning from French President Charles de Gaulle, allowed himself to get drawn into a debate with Nikita Khrushchev over the future of Berlin at a Vienna summit, beginning a stalemate that eventually led to the Berlin Wall.

Bill Clinton is an object lesson of exactly the opposite sort; it was his early indecision that frustrated Europeans to the point of fury. During the campaign, he attacked incumbent George H.W. Bush for failing to exert "real leadership" in Yugoslavia; upon winning the campaign, Clinton then stood by as the fighting raged, leading French President Jacques Chirac to announce that the position of leader of the free world was "vacant." Similar waffling would lead to the ignominious U.S. retreat from Somalia and delay any response to the genocide in Rwanda.

Chastened by those failures, Clinton's second term brought one success after another, including a bailout for Mexico, the Good Friday Agreement in Ireland and unprecedented progress on an Israeli-Palestinian deal.

It's hard to know which mistake Obama will make: over- or underconfidence. He has vowed to show more restraint than George W. Bush did, but there are structural reasons to expect trouble. Obama's team, which consists of Democrats who have not seen foreign-policy action in years, will have to get up to speed at a time when new crises, from Sudan to North Korea, are boiling, says Walter Russell Mead of the Council on Foreign Relations. And the early years of any new administration bring ideals into conflict with reality. Witness Bush's wholesale retreat from the brash unilateralism of his first term.

Several areas for conflict with U.S. allies loom. European publics (more than their governments) want an immediate drawdown of U.S. troops in Iraq, which Obama will not provide. He'll also ask for more help in Afghanistan, which is hugely unpopular among Germans. Britain and Canada want Obama to relieve their troops in the violent south. Only France looks likely to respond to Obama's call.