Instead, Trump has been NATO’s loudest critic. He has cast America’s military allies primarily as a drain on the U.S. Treasury, and he has aggressively criticized America’s true friends in Europe—democratic leaders such as Macron and Germany’s chancellor, Angela Merkel—even as he treats Vladimir Putin, Xi Jinping, Kim Jong Un, and other authoritarians around the world with unusual tact. He describes the European Union, whose membership overlaps significantly with NATO’s, as a competitor rather than the close global partner it has been to every recent American president.

The not-so-closely guarded secret at NATO headquarters is allied officials are privately relieved that, rather than holding a full-fledged summit over two days, the leaders are holding just three and a half hours of formal discussions. That limited Trump’s opportunities to blow up the proceedings, as he has done in other major meetings with European and Canadian leaders.

I previously served as U.S. ambassador to NATO under George W. Bush. I also served as a National Security Council staffer under George H. W. Bush, and was lucky enough to observe how a master diplomat like him could further American goals precisely by working skillfully with our allies. Holding a big alliance like NATO together takes patience, tact, a willingness to listen, deep knowledge of the issues, and the self-discipline to refrain from fiery public debate. In London, Trump proved once again that he is incapable of this type of presidential leadership. Senior European officials fear that, in a second term, Trump might seek to end U.S. participation in the alliance altogether. (For the record, I am an unpaid adviser to Vice President Joe Biden’s campaign.)

Stung by Trump’s overt criticism, U.S. allies have begun to reciprocate. Macron caused a real stir in NATO when he told The Economist last month the alliance was effectively “brain dead,” given Trump’s sole emphasis on how much NATO costs the United States and his lack of consultation with France on the withdrawal of U.S. Special Forces from Syria, where France also has soldiers.

Rather than try to mend fences, Trump announced new trade sanctions against France on the eve of the summit. Then, yesterday morning, he told the press that Macron’s comments to The Economist were “insulting” to NATO. Trump’s pious defense of the treaty organization sounded less than sincere, in light of his previous statements.

Nicholas Burns: The lasting lesson of the Molotov-Ribbentrop Pact

Trump’s most egregious mistake, though, was his failure to support clearly and unequivocally the key provision of the NATO treaty, Article V, which calls on member states to come to one another’s defense when attacked. He has had several opportunities to do so but has hedged each time. This is of major concern to allied leaders, who want NATO adversaries such as Vladimir Putin to know beyond a shadow of a doubt that the U.S. and its allies will defend Estonia or Latvia should Russian forces cross their borders.