A top European official on Tuesday laid out a sweeping framework aimed at rearming the continent, trying to fill a void as President Trump retreated from supporting Ukraine and pressured Europe to spend more on its own defense.

The official, Ursula von der Leyen, president of the European Commission, the European Union’s executive branch, set out a broad but vague proposal to ramp up European defense spending by as much at 800 billion euros, or some $843 billion, including a €150 billion loan program to pay for more weapons and technology.

The plans signal an important policy shift at a tenuous moment, as European leaders panic about Mr. Trump’s demands.

While the E.U. has long seen itself as a project built for peace and open trade, the world has abruptly changed around it. “We are in an era of rearmament,” Ms. von der Leyen said on Tuesday from Brussels in a statement to the news media.

Mr. Trump has been pivoting away from Ukraine and toward Russia in recent weeks. The shift intensified this week, when he suspended the delivery of all U.S. military aid to Ukraine, according to senior administration officials.

America’s realignment has left Europe racing to help Ukraine in the short term and to ensure that it is prepared to defend itself from a more aggressive Russia in the long term.