... Some Europeans may view Brexit mainly as a matter of economics, but it is also inescapably a major security concern for the West. The relationship between post-Brexit Britain and the rest of the West cannot be evaluated simply as an internal matter for the EU. Britain may be leaving the EU, but it is not leaving the American-led Western alliance. The implications of a nasty and brutal Brexit for the Atlantic community are too consequential for Washington to ignore. 

The European leaders taking a hard line against Mrs. May underestimate the role the U.K. plays in sustaining American support for Europe. President Trump has waffled in his commitment to NATO as a whole. But for many voters in Mr. Trump’s base—voters who otherwise are indifferent or even hostile to American involvement in European security—the U.K. is in a category similar to Israel: a country whose defense activates feelings of solidarity and honor. There is no other European country whose place in American affections is as deeply rooted and widely felt as the U.K. The perception in the U.S. that an inflexible EU is vindictively punishing Britain for asserting its national sovereignty would have profound and long-lasting consequences for American views of the EU. 

A rigid EU approach to Brexit would strengthen the hands of those in the Trump administration who favor an all-out trade war with Europe and a weakened American commitment to European security. The EU’s approach to Britain will be seen at the highest levels of the administration as a fundamental test—a measure of France and Germany’s commitment to NATO and the trans-Atlantic relationship. 

This does not mean that the U.S. expects Britain to retain a privileged position in the single market. The EU position that nonmembers cannot enjoy the full privileges of membership is both reasonable and just. But if NATO and the trans-Atlantic partnership still mean anything in Europe, a spirit of alliance solidarity needs to prevail over narrow political considerations.

The stronger party can afford to be generous; that has been the spirit that has governed the Western alliance since the 1940s. Both an “America First” Washington and an inflexible Brussels are now in danger of neglecting that truth. Without magnanimity on the part of its core members, the Western alliance will not long endure.