Political polarization plays an increasingly prominent and formative role in American politics. Its effects are visible in how Americans feel about politics, how candidates for office win election, how compromises can (or, more often, cannot) be reached, and how U.S. institutions increasingly face risks in carrying out even ordinary functions, including periodic crises over shutdowns or defaults. Yet the meaning of political polarization for U.S. foreign policy has been overlooked. This report argues that polarization has begun to produce a great effect on how the United States makes policy, including policy toward allied and partner states. Since the effects of polarization began before the Trump administration and will likely outlast it, states dealing with the United States should take note of this trend, and consider particularly whether the risks of becoming too aligned with one or another political party outweigh the benefits of doing so in the short term. This report reviews recent academic political science literature on political parties and polarization; demonstrates why these factors matter to U.S. foreign policy; and applies these lessons to U.S. foreign policy, with a particular emphasis on issues relevant to the Kingdom of Saudi Arabia.