After three years spent trying to deter Russia from destroying Ukraine, European Union leaders grappled on Monday with how to respond to a major ally who appears determined to start a trade war or even seize part of their territory.

The president’s threats to slap duties on goods imported from the European Union are part of a pattern of directing the harshest penalties at the United States’ closest allies.

The European Union must remain united and avoid being naive in the face of threats by U.S. President Donald Trump to levy tariffs on its products, Spanish Economy Minister Carlos Cuerpo said on Monday.