Some worry an unpredictable Trump, at the U.S.-Russia summit, could agree to take the first steps to embolden Russia, such as halting military exercises or agreeing that Crimea, a region of Ukraine annexed by Russia in 2014, belongs to Russia.

Magnus Nordenman, who worked as a defense analyst and a strategic planning consultant for major European defense industry companies, said European allies are “absolutely worried” after hearing Trump disparage allies of the G-7, as well as NATO members’ contributions and seeing him eager to meet Putin as well as North Korean dictator Kim Jong-un.

“There is element of uncertainty in all this,” said Nordenman, now the director of the Transatlantic Security Initiative at the Scowcroft Center for Strategy and Security at the Atlantic Council. “But we all need to take a bit of a breath here…and hope the president is in a good mood when he goes to Brussels.”