Abstract: This article examines the domestic political dynamics behind US president Donald Trump’s “America First” approach to trade and foreign policy, to understand better how long this strain of American economic nationalism might last and what it means for Japan’s national security strategy. The political base for Trump’s trade protectionism and apparent indifference to allies has roots stretching back into American history, but this movement has strengthened in recent years due to a combination of growing economic inequality in the United States, demographic changes, and the impact of fast-paced technology development and economic globalization. These political trends are likely to persist beyond Trump’s presidency, although some potential negative effects on Japan and the US-Japan alliance in the medium-to-longer term can be mitigated by Japan’s proactive foreign policy and other steps. The alliance still offers a great deal of value to both countries—now and into the future—but Japan should consider a slightly larger global leadership role in concert with others, even as the allies work to enhance their continued cooperation on shared interests.