BRUSSELS — European Union officials shocked by a week of mixed messages from the Trump administration on the future of European integration may decide to treat the United States — at least in part — as a politically hostile entity that could limit the security and cooperation relationship that has underpinned the postwar Atlantic alliance, according to multiple diplomats and security officials in Brussels.

The Trump campaign’s public support for Brexit and apparent ideological affinity with Marine Le Pen’s anti-EU National Front as well as statements by key White House advisers like Steve Bannon has left European intelligence, security, and diplomatic officials considering a radical change in a relationship that has long underpinned security and stability in Europe.

“We’re spending far too much time in meetings trying to understand this administration’s approach to the 50-plus-year-old European integration project,” said one Brussels-based diplomat who did not want his country named criticizing the US during a fraught period of transition.

“The underlying theme in these meetings is determining if America plans to remain a steadfast ally of the EU during a period where a resurgent and mischievous Russia is making common cause with the populist right wing in Europe and abroad to weaken, if not eliminate the EU,” the diplomat said.

“How are we supposed to work with the country supposed to be our closest ally on issues [such as] Ukraine, terrorism, the refugee crisis, and tensions between Serbia and Kosovo while wondering about the rhetoric from our ally calling for our elimination?" the diplomat added. "It’s hard to be allied with someone who wants to destroy you."

The administration’s decision to send the generally well-regarded US secretary of defense, Jim Mattis, to Europe over the last week to reassure allies about the US commitment to both the EU and NATO started off well, say multiple EU observers, until his speech to NATO threatened a withdrawal of American support on defense issues unless spending is increased.

"I owe it to you to give clarity on the political reality in the United States and to state the fair demand from my country’s people in concrete terms,” Mattis told European defense officials. “America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to this Alliance, each of your capitals needs to show support for our common defense.”

Although US pressure to increase defense spending to the mandated 2% of GDP has been a longtime policy, the language used by Mattis, broadly considered an experienced and thoughtful policy hand, shocked some observers because it contradicted the tone of a private meeting between Mattis and German Defense Minister Ursula von der Leyen on Feb. 10 at the Pentagon, according to accounts from EU observers and reports from the German media.

“Mattis was speaking from a script that felt like it was written by Bannon,” said an EU analyst who was in the audience in Brussels. “After he had gone to great lengths in his meeting with von der Leyen to reassure Europe that the adults were in charge of security policies.”