Mattis, a former US Marine Corps General and former NATO Supreme Allied Commander of Transformation, who has been picked by Donald Trump to head the US defense establishment, put before the NATO leaders the US demand in unequivocal terms.
British Defense Secretary Michael Fallon agreed with Mattis, saying that the organization "must become more agile and allies must spend more on defense".
"Frankly, this has no impact", Mattis told reporters on his flight to Brussels on Tuesday. Obviously, I haven't changed what I'm heading there for. "It doesn't change my message at all, and who is on the president's staff is who I will work with".
Still, allies must do more, Mattis said. It's fine to shrug here and say "if they can't or won't pay, let 'em enjoy Russian occupation", but Russian expansionism westward comes with costs for the United States too. "Mattis is a military guy; you don't embarrass the boss".
Stoltenberg also insisted the Flynn scandal was not a further cause for concern for the alliance, which has underpinned transatlantic security since the aftermath of World War II.
He also said that the countries which don't now meet their commitment had "expressed ambitions, plans" to increase defence spending. Now, it appears the administration has taken on a new attitude toward the alliance.
The feeling among diplomats and ministers here is that Gen Mattis may agree with this, but no one knows what President Trump will do: the future vista from this landmark summit is one shrouded in uncertainty and trepidation.
"We are training Iraqi officers and supporting the Counter-ISIL Coalition with AWACS surveillance planes", said Stoltenberg. Mattis told his counterparts that countries that don't meet NATO's military spending recommendation of 2% of gross domestic product should adopt plans to do so.
During his election campaign and in an interview since his victory, Trump demanded that European nations contribute a larger share to their defense.
The warning came on Mattis' first visit as Pentagon chief to NATO's headquarters in Brussels, where, in response, allies emphasized the need to accelerate efforts to increase their own defense expenditures.
Meanwhile, NATO has sent alliance troops to Russia's borders in an attempt to deter Russian Federation from taking any further action toward neighboring countries Crimea and the Ukraine. "Americans can not care more for your children's security than you do".
"When America retrenches and retreats, it leaves behind a vacuum, and that vacuum is filled by bad guys", Rasmussen said.
Many in Europe see elements in the Trump administration as having an in-built antipathy towards multilateral institutions.
This is Mattis' second global trip as Trump's Pentagon chief.
He told the assembled defense leaders that North Atlantic Treaty Organisation, with its members' shared commitment, will remain what President Dwight D. Eisenhower described as a "valuable, necessary, and constructive force".