The idea of a European Defense Union is gaining ground across Europe. But so long as NATO continues to dominate Europe’s security, the prospect of building its own effective defense union will remain elusive. To become sovereign in defense matters (and more generally), Europe must terminate NATO — a prospect as unlikely as it is necessary. Mark Rutte, the former Dutch prime minister who is now NATO’s secretary-general, recently let slip a truth that drew gasps from across Europe.

Mark Rutte is wrong to quash talk of one. The risks of the alliance unravelling are too great to ignore.

The idea of a European Defense Union is gaining ground across Europe. But so long as NATO continues to dominate Europe’s security, the prospect of building its own effective…