Europe Races to Craft a Trump-Era Plan for Ukraine and Defense

European leaders find themselves plunged into a hostile new era of less cooperation from the United States, a greater threat from Russia, and an increasingly uncertain outlook for Ukraine. On Thursday, they will gather in Brussels to try to figure out what to do about that.

They will need to walk a fine line: Figuring out how to support Ukraine and bolster their own defenses while avoiding further alienating their tempestuous allies in Washington.

The specially called and much anticipated gathering will include the heads of state or government from the European Union’s 27 member countries and is the latest in a series of quickly arranged summits focused on defense.

They are expected to discuss how to finance a ramp up of European military spending, hoping to make the continent better armed to deal with Russia without as much backup from across the Atlantic. They will also review what a peace plan for Ukraine might look like, and how they can support it.

In some ways, the meeting marks the start of a new chapter for the European Union. Created to foster cooperation and peace, the bloc is being forced to contemplate its role in a world riven by conflict and animosity, even among allies.

The pressing question is whether the E.U., with its consensus-focused, clunky structure, can adapt itself quickly enough to ensure that Europe doesn’t get left behind as Mr. Trump rapidly changes the global order.