During his second term, Donald Trump has railed against the United States’ closest allies. He has imposed tariffs, threatened to upend security commitments, and openly challenged the borders of Canada, Panama, and Greenland. Historians often look to the past for insight about the present and future. But although alliances have collapsed for many reasons over past centuries, Margaret MacMillan argues in a recent essay for Foreign Affairs that Trump’s current behavior toward allies has little precedent. His approach, she writes, “does not suggest a clever Machiavellian policy to enhance American power; rather, it shows a United States acting against its own interests in bewildering fashion, undermining one of the key sources of that power.” A renowned historian and professor emeritus of international history at Oxford University, MacMillan is one of the greatest chroniclers of the grand alliances of the twentieth century and the world wars they fought. She joined Editor-at-Large Hugh Eakin on August 18 to discuss the normalization of conquest and the war in Ukraine, how U.S. allies are calculating their next steps, and what the United States’ approach to its alliances will mean for the future. You can find sources, transcripts, and more episodes of The Foreign Affairs Interview at https://www.foreignaffairs.com/podcasts/foreign-affairs-interview.