Vladimir Putin plays the long game. For decades, the Russian leader has worked to subjugate Ukraine. For decades, he has also cultivated ties with Donald Trump. Now, success at the latter endeavor is allowing Putin to recover from failures at the former: after nearly five years of thwarted Russian objectives in the war on Ukraine, the U.S. president is helping him win. By easing sanctions on Russia, cutting military and economic assistance to Ukraine, disrupting intelligence cooperation, and pressuring Ukrainian President Volodymyr Zelensky to accept Putin’s war aims, Trump is making Russia’s successful annexation of substantial Ukrainian territory and de facto control over Kyiv’s future decisions more likely than at almost any point since February 2022.

Since Putin launched his full-scale invasion of Ukraine, battlefield momentum has swung back and forth between the two sides. Russian forces had the advantage at the start.…