President Trump on Wednesday followed through on his threat to impose a 50 percent tariff on nearly all goods arriving from India, leveling one of his most punitive tariffs at a country with deep ties to the United States.

The 50 percent rate, half of which is punishment for India’s buying Russian oil, is expected to damage many Indian exporters that collectively employ millions of people. The move could rupture America’s expanding economic relationship with India, where two-thirds of the largest U.S. corporations have offshore operations. The tariff also undermines the stability of billions of dollars of foreign investment in India’s stock market, the world’s fourth largest.

The extraordinary levy puts India at a disadvantage in the new trading order Mr. Trump set in motion when he announced tariffs on dozens of countries in April.

Mr. Trump has declared a cease-fire with China, which ran a more than $1 trillion global surplus last year and is considered the principal antagonist in his trade war. In that context, the 50 percent tariff on India risks undermining a key strategy used more by American importers in recent years to shift production to India to lessen their dependency on Chinese factories.

Recent weeks have brought a turnabout for India, which was confident that its importance to the United States and the rapport between its prime minister, Narendra Modi, and Mr. Trump would earn it a reprieve. But now India is alone with Brazil — led by a leftist president who has antagonized Mr. Trump directly — with 50 percent tariffs, higher than any other country.