America has not been seriously committed to the defence of Europe for many decades. After the Cold War, the US remained the leading power in NATO and championed the Alliance's eastward enlargement. But American leaders expected that the costs and risks to the US would remain extremely low. Rather than decide that new allies were worth defending if attacked, they largely wished the problem away. Now that Europe's security environment has deteriorated and America's unipolarity is gone, NATO must change. Because Europe has the resources to outmatch Russia and a greater interest in defending its territory than the US does, it should become capable of and responsible for defending itself. European countries should plan to replace most US military personnel and infrastructure in Europe over the next decade. The Trump administration should stay in NATO, help European allies that step up, and encourage Europe to build its defence-industrial base.