The European Union and the United States agreed on Sunday to a broad-brush trade deal that sets a 15 percent tariff on most E.U. goods, including cars and pharmaceuticals, averting what could have become a painful trade war with a bloc that is the United States’ single biggest source of imports.

President Trump said that the European Union had agreed to purchase $750 billion of American energy, which Ursula von der Leyen, the president of the E.U.’s executive branch, told reporters would be spread out over three years.

The 27-nation bloc also agreed to increase its investment in the United States by more than $600 billion above current levels, Mr. Trump said, adding that the European Union would buy military equipment. A senior U.S. official said that those investments would include pharmaceuticals and the automotive industry, among others.

The two sides also agreed to drop tariffs to zero on a range of goods, including aircraft, plane parts, certain chemicals, certain generic drugs, semiconductor equipment and some agricultural products, Ms. von der Leyen said.

Altogether, while it was clear that major details still needed to be hammered out, the framework seemed likely to permanently reshape the trading relationship between two of the world’s largest and most interconnected economies.

The agreement will “rebalance, but enable trade on both sides,” Ms. von der Leyen said as she sat next to Mr. Trump and the leaders made the announcement.