However, because only five of the member nations now meet the spending target - 2 percent of their gross domestic product must be spent on defense - required for membership, Mattis issued a strong directive to alliance members regarding the need for a renewed focus on meeting the financial requirement for membership.
Mattis's visit to North Atlantic Treaty Organisation, his first trip to Europe since being sworn in last month, comes amid a growing scandal at the White House following the resignation of Trump's national security adviser Michael Flynn.
Mattis, though, said in the meeting that it's now a "governmental reality" that the United States would lose patience with members being slow to meet the spending target. "We have to continue to increase defense spending across Europe and Canada. Americans can not care more for your children's future security than you do", he said, according to a text of his intervention in the meeting.
Trump has praised Russian President Vladimir Putin, and USA national security advisor Michael Flynn was viewed as a leading advocate of warmer relations with Russia. The potential interdiction seemed in keeping with recent instructions from Mr. Trump, reinforced in meetings with Mr. Mattis and Secretary of State Rex W. Tillerson, to crack down on Iran's support of terrorism.
"This is clearly a demonstration of USA determination to stand with Europe in these trouble times", he said.
The blunt threat cast an ominous cloud over a meeting that began with NATO Secretary-General Jens Stoltenberg warmly welcoming Mattis back to Brussels, where he had previously served two years as supreme allied commander for transformation.
The announcement doesn't really come as a surprise following reports yesterday that Mattis would focus on this issue.
Sen. Schumer calls for investigation into the former National Security Adivser
Those facts and the Department of Justice's own rules disqualify Attorney General Sessions from running this investigation.
"We remain open to opportunities to restore a cooperative relationship with Moscow, while being realistic in our expectations and ensuring our diplomats negotiate from a position of strength", he said. "We have made the first steps, but there is still a lot to do".
"Flynn made people nervous, both because of his closeness to Russian Federation and being part of that political inner circle that doesn't seem to be very stable", said Mark Cancian, a former Marine colonel who served with Mattis in Iraq and is now a defense analyst at the Center for Strategic & International Studies.
Calling the alliance "a fundamental bedrock" for the United States, he tried to reassure allies concerned about President Donald Trump's commitment to the bloc.
The United States for years has called for European allies to spend 2 percent of economic output on defence, he said.
New Nato figures show that Britain is meeting its target of spending 2% of its GDP on defence, the military alliance has announced.
"We are not willing, however, to surrender the values of this alliance nor let Russian Federation, through its actions, speak louder than anyone in this room", he said.
Of NATO's 28 members, just the U.S., Greece, Poland, Estonia and the United Kingdom met the target.