The European Union has been following tried-and-true rules of global commerce as it tries to negotiate with the Trump administration to avert painful tariffs on cars, pharmaceuticals and just about everything else.

The problem? President Trump is ripping up that rule book.

Mr. Trump announced in a Truth Social post on Friday morning that he is recommending a 50 percent tariff on European imports as of June 1, claiming that the bloc’s trade barriers, taxes, corporate penalties and other policies had contributed to a trade imbalance with the United States that was “totally unacceptable.”

“The European Union, which was formed for the primary purpose of taking advantage of the United States on TRADE, has been very difficult to deal with,” Mr. Trump wrote, adding, “Our discussions with them are going nowhere!”

The surprise announcement comes after months of back-and-forth talks between the two enormous economies that have made, as Mr. Trump suggested, limited headway.

European officials have approached negotiations as though they are reasoning with an ally. But they have met with a Trump administration that sees this less as a chance for two geopolitical friends to seek a mutually beneficial solution — and rather as an opportunity to pressure a commercial rival into making concessions.

Mr. Trump has imposed round after round of tariffs on the 27-nation economic bloc and the world since taking office in January. He has hit sectors like steel and aluminum and cars with specific tariffs, while also threatening to place higher across-the-board levies on most American trading partners. But back in April, he announced that he would pause that latter category for a 90-day period as countries negotiated deals.