The two major parties in America have switched positions many times on the question of how much the United States should intervene in foreign affairs. Are these ideological changes simply the product of historical contingency, or are there structural factors at work that can help explain these developments? This chapter finds that change in party control of the presidency can help explain change in party ideologies with respect to foreign policy. Examining ten cases in American history from the time of Theodore Roosevelt to the present, this chapter shows that parties in long-term control of the presidency tend to change their ideology in ways that call for more foreign intervention, while parties in opposition to the presidency tend to change their ideology in ways that call for less foreign intervention.