Donald Trump brazenly chastised NATO leaders to their faces for not paying their fair share to protect the alliance, adding that the current situation is "not fair".
Speaking from the organisation's new headquarters in Brussels (a city he once called a hellhole), his speech garnered a mix of stony faces and suppressed laughter from the heads of state assembled to listen.
Unfortunately for Trump, he did nothing to change the opinion of many that he fundamentally misunderstands how NATO works.
Here's what he got wrong.
The 28 member nations, plus soon-to-join Montenegro, will renew an old vow to move toward spending 2 percent of their gross domestic product on defense by 2024. Only five members currently meet the target: Britain, Estonia, debt-laden Greece, Poland and the United States, which spends more on defense than all the other allies combined.
Moreover, the White House had sent recent signals that the United States would stay in NATO's mutual defense pact, known as Article 5, which had been invoked just once before: after the terror attack of September 11, 2001. But Trump made no mention of the Article 5 commitment as he spoke next to a new monument centered on steel from the crumpled World Trade Center.
Asked about Trump not explicitly affirming U.S. support for Article 5, White House press secretary Sean Spicer said: "It goes without saying. His presence at this event underscores our commitments and treaty obligations."