BRUSSELS — European leaders have long understood that President Trump is an unreliable ally, subject to loud tantrums, abrupt shifts and sudden whims. They have worried about his ambivalence toward NATO, resented his personal attacks and bristled at his use of trade policy and economic sanctions to restrict their companies and markets.

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.