Update: This article was posted before Trump’s speech, with the headline “Europe’s leaders desperately want Trump to say these two words at today’s NATO summit.” He didn’t say them.

Not “the wall.” Not “fake news.” Not “Russian hacking.” In fact, Europe’s leaders will even grin and bear it if US president Donald Trump doesn’t publicly say “climate change” as he visits Brussels for a NATO meeting today.

The two words he really, really does need to say, though, are “Article Five.” Specifically, that he supports this, the most important clause in NATO’s treaty, which decrees that “an attack against one ally is considered an attack against all.”

Trump has come a long way in his views on NATO since stepping up to the presidency—having called it “obsolete” on the campaign trail, he reversed his position last month and said that’s “no longer” the case. (The reason, ostensibly, being that NATO had changed in that time, though no NATO observers can detect any difference.)

As much of a relief as it is that the US is no longer threatening to quit the alliance, Trump will still be undermining NATO if he doesn’t come out in favor of Article Five. The whole point of the alliance was to deter attacks by the Soviet Union against the Western European allies—and Article Five meant that any attack on a small European country could potentially be met with the full force of the United States military.

After the fall of the Soviet Union, the article did indeed come to seem relatively obsolete—it has only ever been invoked once, the day after the 9/11 attacks. But it has returned to the forefront since Russia’s annexation of Crimea in 2014, as Moscow has tried to rattle the alliance that it sees as the primary obstacle to its regional influence. Since Crimea there’s been much concern that Russia’s “little green men” could just as easily pop up in the tiny Baltic states, on the Russian border.

NATO leaders have reminded Russia at every opportunity that any meddling within their borders will incur the wrath of the whole alliance. But that warning will be hollow if the US president doesn’t back it up by committing fully to Article Five.