Trump didn’t say the one thing about NATO he was supposed to say

European leaders had high hopes that President Donald Trump would finally endorse Article 5 of NATO — the keystone principle that an attack on one member is an attack against all — during a hotly anticipated speech at the alliance’s headquarters on Thursday. But after a speech in Brussels with those allies, those hopes have been dashed.

After bashing “23 of 28 nations” in NATO for failing to live up to their defense spending obligations, Trump made no mention of American commitment to the promise of mutual defense that underpins the organization.

It’s difficult to overstate how deafening his silence on the matter was to European leaders. As with many of his foreign policy stances, Trump has often been inscrutable and inconsistent on his position on NATO. On the campaign trail he slammed it as “obsolete” and suggested that his commitment to Article 5 was conditional, when he said that the US would only defend Baltic members of NATO against a Russian invasion if they increased their spending commitments to the alliance.

For a moment, it looked like Trump’s feelings toward NATO were on the upswing. In April he declared that “it was no longer obsolete.” And his administration’s warm overtures toward Russia — which NATO is meant to counter and contain the power of — were quickly tainted by confrontations over the war in Syria. As the Brussels speech on Thursday approached, an official from his administration told the New York Times that the president would finally endorse Article 5, bringing the administration fully back to the status quo that’s prevailed since NATO was founded.

But that didn’t happen. It’s possible that the source was wrong, that the speech was tweaked again as it was finalized by other advisers who are skeptical of NATO like chief strategist Steve Bannon, or that Trump himself decided to nix it at the last second — perhaps even during the speech itself.

European leaders probably found the timing of Trump’s refusal to pledge to uphold Article 5 to be particularly galling. Trump is unveiling a memorial to the victims of 9/11 while in Brussels — the only event that has caused NATO to invoke Article 5. The NATO alliance collectively fought the war in Afghanistan which was launched in the wake of those attacks on the US.

In the meantime, NATO is stepping up its commitment to US-led initiatives. On Thursday, NATO decided to officially join the US-led coalition against ISIS, although it did not pledge the use of combat troops as part of that commitment.

The strength of NATO is particularly relevant these days in light of terrorist violence and a resurgent Russia. The continent is currently enduring a fairly steady wave of terrorist attacks — most recently, a suicide bombing this week at a concert in Manchester, England, that killed 22. And Russia’s belligerence in recent years, with its meddling and territorial expansion into Ukraine and its involvement in the Syrian civil war, has its neighbors on edge about the possibility of more expansionism. Trump’s deliberate decision to keep mum on his stance on Article 5 broadcasts a clear message to the West’s military adversaries: NATO is not stable, the US cannot be relied upon to pull its weight on behalf of its allies.

So what’s Trump’s game here? What does he actually hope to accomplish? Most likely he thinks that being coy on US commitment to Article 5 will make member states who lag in their defense spending pick up the slack.

The US has long complained to its allies that they need to spend more money on defense, and they generally haven’t done it, so the reasoning behind Trump’s hardball negotiating is understandable. But it also comes at a potentially high cost: It makes allies deeply nervous about the limits of US support, and it will empower Russia.