President Donald Trump's press conference with German Chancellor Angela Merkel following the pair's meeting was largely positive, but there was one thing Trump didn't address that could be a problem, according to Nick Burns, former U.S. ambassador to NATO.
"If you were listening for U.S. support for the EU, you didn't hear it from President Trump. Europeans are worried about that," Burns said in an interview with "Power Lunch" on Friday.
"He's the first American president since Truman to be so negative about the project of building European unity. That's a problem."
"I reiterated to Chancellor Merkel our strong support for NATO, as well as the need for our NATO allies to pay their fair share for the cost of defense," Trump said at a joint White House press conference following the meeting.
Merkel said she was "gratified to know" how "important" Trump feels NATO is.
Wendy Sherman, former undersecretary of state for political affairs at the U.S. State Department, said it's important that the United States works with the alliance to keep Europe "whole and free."
"It is the most successful security alliance that the world has ever seen. It has kept Europe at peace. It has helped outside of that theater as well," she told "Power Lunch."
And the U.S. can't discount the fact that Europe is critical to trade and the development of jobs in the U.S., she added.
"We cannot neglect the reality that Germany has created a lot of jobs in America and that's what President Trump says he's all about," said Sherman, now a senior counselor at Albright Stonebridge Group.
Burns, now a professor at the Harvard Kennedy School, thinks the most important thing to come from the meeting is the fact that the two leaders establish some measure of familiarity and trust.
"Angela Merkel is going to be his most important partner on a variety of economic, political and strategic issues, most notably the effort to contain Vladimir Putin," he said.
— CNBC's Jacob Pramuk contributed to this report.