Donald Trump’s first major statement about NATO came in March 2016 in an interview with The New York Times. He said the alliance was obsolete for several reasons. The first was that Russia no longer posed the threat the Soviet Union did. The second is that NATO was not focused on counterterrorism. The third is the financial cost of NATO to the United States.

In the year that followed, Trump doubled down on this critique, particularly on terrorism and burden-sharing. He repeatedly argued that NATO members must pay up if they want to receive U.S. protection. In July 2016, he said, “I want to keep NATO but I want them to pay. … I don’t want to be taken advantage of…We’re protecting countries that most of the people in this room have never even heard of and we end up in world war three … Give me a break.”

After taking office, Trump claimed credit for a shift on burden sharing and counterterrorism. He told a joint session of Congress that the “money is pouring in. Very nice.” When he met the NATO Secretary General Jens Stoltenberg in April, he said: “I complained about that a long time ago, and they made a change—and now they do fight terrorism. I said it was obsolete. It’s no longer obsolete.”

The crucial nuance is that Trump did not say that NATO’s original mission of countering Russian power in Europe is no longer obsolete. Indeed, he has never acknowledged this. The closest he has come is a general statement of strong support with caveats attached for burden sharing and counterterrorism. Trump’s failure to endorse Article Five is not an oversight. Members of his cabinet have unsuccessfully tried to insert this language into his remarks, including at his meeting with Stoltenberg.

Compare this with Trump’s predecessor. President Obama went to Estonia in 2013 and said:

“I say to the people of Estonia and the people of the Baltics, today we are bound by our treaty Alliance. We have a solemn duty to each other. Article Five is crystal clear: An attack on one is an attack on all. So if, in such a moment, you ever ask again, ‘who will come to help,’ you’ll know the answer—the NATO Alliance, including the Armed Forces of the United States of America.”

One can criticize Obama’s Russia policy, but he understood the sacred importance of Article Five.

Others in the Trump administration have attempted to compensate for the president’s reticence. Defense Secretary James Mattis and Vice President Mike Pence explicitly endorsed Article Five of the NATO Treaty at the Munich Security Conference in February of this year. Mattis called it a “bedrock commitment.” Pence said it is “one of two core principles that are central to” NATO’s mission and that U.S. support for the alliance is “unwavering.”

Mattis and Pence have helped bolster U.S. credibility on NATO, as has the continuing deployment of U.S. forces and assets to Eastern Europe which began under the Obama administration. But ultimately the decision to uphold U.S. commitments lies with the commander-in-chief. He alone decides whether and how the United States will respond to a Russian attack on a NATO member.

If NATO is to be as effective in the future as it has been in the past, it needs a firm commitment from President Trump. He has already moderated his position on U.S. alliances. When he stands beside a piece of the wreckage of the World Trade Center, he should complete his journey and say what every postwar president has said—the United States will stand by its allies in NATO by honoring its commitments under Article Five.

This article appears courtesy of the Brookings Institution.