Until now, Europeans have done little except complain about him. But Mr. Trump’s recent actions in Syria and Ukraine may change that.

The more optimistic now argue Mr. Trump’s betrayals in those conflicts are of a different category of seriousness, and may accelerate what has been a slowly building process of European integration and peeling away from the United States. Others are not so sure.

But there is agreement that Mr. Trump has destabilized Europe’s near neighborhood in a major, even fundamental, way that requires a unified response, if only Europeans can come together…

“Europeans have put themselves in the position of being dependent on an undependable president,” said Robin Niblett, director of Chatham House, the Royal Institute of International Affairs in London.