US President Donald Trump has repeatedly cast doubt on Washington’s commitment to defend Nato territory. His threat to use military force against Nato member Denmark to gain control of Greenland was later dropped but will not be forgotten by his allies. America’s security umbrella could soon come with ideological or economic conditions that Europeans are no longer willing to fulfil. Even if the US remains nominally committed to defend Europe, Russia and other adversaries may try to exploit perceived chinks in the alliance to coerce Nato’s European members with nuclear threats.