BRUSSELS — President Trump seems determined to upend 70 years of established American foreign policy, especially toward Europe, which he regards as less ally than competitor.

The Trump turnabout has set off a fervent search on both sides of the Atlantic for answers to hard questions about the global role of the United States, and what a frazzled Europe can and should do for itself, given a less reliable American partner.

The German foreign minister, Heiko Maas, speaking before a conference of all Germany’s ambassadors last month, argued for a stronger European foreign and defense policy in the face of a suddenly uncertain future.

“The rules-based international order” is eroding in a world where “nothing can be taken for granted any more in foreign policy,” he said.