In the wake of the financial crisis, the United States is no longer the leader of the global economy, and no other nation has the political and economic leverage to replace it. Rather than a forum for compromise, the G-20 is likely to be an arena of conflict.

China, which invests heavily in Iran's energy sector, is the linchpin of the sanctions regime against Iran. If Washington wants to prevent Tehran from acquiring nuclear weapons, it must transform Beijing from a silent, subordinate partner to a vigorous ally.

What does rise of the Tea Party movement mean for U.S. foreign policy? Since today's populists have little interest in creating a liberal world order, U.S. policymakers will have to find some way to satisfy their angry domestic constituencies while also working effectively in the international arena.