This week’s North Atlantic Treaty Organization meeting in Brussels promises to be a most unhappy reunion. President Trump likely will lash out at countries spending less on defense than the alliance’s benchmark 2% of gross domestic product. The atmosphere won’t be conducive to discussions about emerging threats. Yet that doesn’t mean the end of the world—or the end of NATO. On the contrary, now is an excellent opportunity for European countries to team up in smaller groups that complement the alliance.

“We need to think asymmetrically...