Defense Secretary Jim Mattis leveraged the threat from Russia on Wednesday to prod NATO allies into spending more on common defense.
"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," Mattis told NATO defense ministers meeting at alliance headquarters in Brussels.
"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," he said.
The U.S. has long pressed the allies to spend at least two percent of their gross domestic products on defense, but only five of NATO's 28 members currently meet or exceed the mark -- the U.S., Britain, Poland, Greece and Estonia.
Mattis urged NATO members falling short of the two percent guideline to come up with plans and fixed dates for meeting the commitment.
Increased defense spending by the allies would bolster U.S. support for the alliance despite campaign statements from President Donald Trump questioning the viability of NATO and calling it "obsolete," he said.
"I join you today representing America's commitment and President Trump's strong support for our alliance," Mattis said, but the support came with a caveat.
Frustration with the allies' failure to meet the two percent mark is now a "governmental reality" in the Trump administration, Mattis said.
"No longer can the American taxpayer carry a disproportionate share of the defense of Western values," he said. "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 threat from Russia and the designs of President Vladimir Putin are clear, Mattis told the ministers.
"How our defensive alliance responds to threats and provocations is not lost on any nation to our east, nor on its leader," he said. "While the U.S. and the alliance seek to engage Russia, we must at the same time defend ourselves if Russia chooses to act contrary to international law."