Defense Secretary James Mattis on Wednesday issued an ultimatum to NATO while meeting with member nations for the first time in Brussels: Pay more, or the U.S. will “moderate its commitment” to the organization.

“I owe it to you all to give you clarity on the political reality in the United States, and to state the fair demand from my country’s people in concrete terms,” he said during a closed-door meeting, according to remarks provided to reporters traveling with him. “America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to the alliance, each of your capitals needs to show its support for our common defense.”

During his presidential campaign, President Donald Trump had repeatedly called on NATO members to pay more for their self-defense in Europe. Only five of 28 member nations had met a requirement to spend at least two percent of their GDP on defense: the United States, the United Kingdom, Estonia, Poland, and Greece.

NATO members France and Germany currently spend 1.78 percent and 1.19 percent respectively, according to the NATO figures.

According to the Washington Post, Mattis recalled that when he was NATO’s supreme allied commander of transformation from November 2007 to September 2009, then-Defense Secretary Robert Gates warned NATO nations that America “would lose their patience for carrying a disproportionate burden” of the defense of allies.

Mattis said that impatience is now a “governmental reality.”

“No longer can the American taxpayer carry a disproportionate share of the defense of Western values,” he added. “Americans cannot care more for your children’s security than you do. Disregard for military readiness demonstrates a lack of respect for ourselves, for the alliance and for the freedoms we inherited, which are now clearly threatened.”

The meeting comes ahead of a visit to NATO by Vice President Michael Pence on Monday.

Before the closed-door meeting, Mattis called NATO a “fundamental bedrock.”

“The alliance remains a fundamental bedrock for the United States and for all the trans-Atlantic community,” he said.

During his confirmation hearing, Mattis called the alliance “the most successful military alliance, probably in modern history, maybe ever.” His first calls to foreign counterparts included NATO members, as well as NATO Secretary General Jens Stoltenberg.

But he also said in responses provided to the Senate Armed Services Committee in advance of the hearing, “member states must share the burden of common defense, and meet or exceed the commitment to reach the two percent defense spending goal that their leaders set at the NATO summit in 2014.”