Severe party conflict, not a high-minded suspension of politics, now prevails “at the water’s edge.” Democrats and Republicans fight pitched battles over foreign affairs. But are the two parties polarized in their substantive preferences on foreign policy, or mainly jockeying for partisan advantage? Are they polarized on foreign policy less sharply than on domestic policy? What are the sources of party polarization over foreign policy? Using a new measure of senatorial foreign-policy preferences from 1945-2010, we explore party polarization over foreign policy. We find that foreign-policy preferences have had varying relationships with party politics and general ideology. Since the 1960s, however, the parties have become increasingly polarized on foreign policy. Using a multilevel analysis, we show that foreign-policy polarization has developed in response to partisan electoral rivalry, foreign-policy events, and general ideological polarization. The analysis indicates an increasing influence of domestic politics on foreign policy.