How does US foreign policy under a unified government, when the White House and Congress are controlled by one party, differ from that under a divided government, when the White House and two chambers of Congress are controlled by different parties? My dissertation answers the question from three dimensions of US foreign policy: 1) the amount of attention that politicians give to foreign policy; 2) the policy choice between multilateralism and unilateralism; and 3) the policy outcome. The theory is based on three assumptions grounded in common knowledge of American politics: 1) both the politicians’ attention span and institutions’ agenda space are limited; 2) addressing domestic issues yields more electoral returns than addressing foreign affairs; 3) a unified government is a hard-won and fleeting opportunity for the majority party to shape policy.