On December 1, Andrés Manuel López Obrador was sworn in as Mexico’s president, beginning a new era in relations between the United States and its southern neighbor. The two nations share a complex set of overlapping interests on trade, security, and immigration, which have drawn them closer together in recent decades. But presidents Trump and López Obrador were elected on vows to put their nations first: to win deals against the forces of globalization and the political establishment, and to wrest better terms from NAFTA, while the US is taking an ever harder line on immigration and border security. As right wing and left wing populist leaders face each other across the Rio Grande, what are the prospects for future cooperation?