Pentagon spokesman Capt. Jeff Davis could not confirm if Mattis would have one-on-one meetings with all of the worldwide counterparts, in part because the secretary's schedule was still in flux. Obviously, I haven't changed what I'm heading there for.
"Whomever is on the president's staff is who I will work with", he said.
"Regardless of language, the most important thing is that we increase defence spending and that is exactly what we are doing", Stoltenberg said when asked about NATO's response to Trump's calls for it to do more to share the burden with Washington.
Mattis said he would also reassure allies that the ouster of Trump's national security advisor, Michael Flynn, would not affect his job leading the Pentagon. The other member nations have pledged to increase their defense spending to meet the target beginning in 2024. "He has a lot of experience as a warfighter and as a combatant commander, and understands the importance of having allies and intelligence, integrating that with combat operations", Cancian said.
"We need to be clear that the response to worldwide terrorism cannot be led by NATO, but it can a part of that", a second senior European NATO diplomat said. "Mattis is a military guy; you don't embarrass the boss".
The U.S., which is easily NATO's most powerful member, now spends 3.61 percent of GDP to defend the alliance, the Associated Press reported. Trump said he was prepared to tell those countries: "Congratulations: You will be defending yourself".
Amid continuing questions over the new administration's attitude towards Russia, General Mattis described the events of 2014 - when Russian forces annexed Crimea - as "sobering".
Mattis, a former North Atlantic Treaty Organisation commander, said it's important for the alliance to remain strong.
"We need to be very wary of what Russian Federation is up to".
But he is also expected to stress Trump's demand that member countries increase their military spending.
US President Donald Trump set alarm bells ringing in the Western military alliance during his election campaign a year ago when he suggested that other members were not paying their way.
Speaking in Brussels, the alliance's secretary general Jens Stoltenberg unveiled figures showing that European members and Canada had between them increased defence spending by 3.8% above inflation a year ago.
Britain's sentiment comes at it prepares to leave the European Union. "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. The Nato meeting is followed by Europe's premier annual security event - the Munich conference - after which the USA vice-president himself will also be stopping by at Nato.
"When America retrenches and retreats, it leaves behind a vacuum, and that vacuum is filled by bad guys", Rasmussen said.