Corbyn’s Labour Party has never gotten the chance to prove its competence or incompetence in government. But it too is weakened by incoherence. Corbyn remains instinctively opposed to international institutions such as the European Union at a time when many in the Labour Party are passionately opposed to Brexit. As a result, Corbyn has failed to take a clear stance on the most important political issue of the day, trying to stay true to his euroskeptic instincts without alienating his increasingly Europhile base—and succeeding only in alienating both.

Perhaps Corbyn wasn’t so popular because he promised to nationalize the railways or declared his lasting solidarity with Fidel Castro and Nicolás Maduro, but because he could credibly claim to be a pain in the establishment’s neck. And perhaps Sanders did so well in 2016 not because Democratic primary voters were desperate for an avowed socialist, but because he was the only real alternative to Clinton.

In the wake of a massive economic crisis, the far left was given a rare opportunity to move from the fringes to the mainstream by channeling the anti-establishment fervor of ordinary voters. The past years have shown that the task of sustaining that initial surge of support is far harder than the movement’s most bullish cheerleaders recognize. If American leftists want to fare better than their European comrades, they urgently need to take note.