President Trump all but declared economic war against India on Wednesday, threatening to add a 25 percent punitive tariff for India’s purchases of Russian oil on top of a 25 percent tariff he announced last week. In his second term, Mr. Trump had been expected to marshal India as a friendly counterweight to the challenge posed by China.

But added together, the 50 percent tariff paints India as a political enemy, putting it in the company of Brazil, whose leftist president sparred with Mr. Trump when the country was threatened with a similarly punishing tariff rate. The crisis between India and the United States suddenly looks much bigger than the terms of trade.

The onslaught against India started on July 30, when Mr. Trump declared that India’s economy was “dead.” Until that point, his administration had been angling to reduce India’s trade barriers, but said nothing about its two years of buying Russian oil at a wartime discount.

Before the shock of Mr. Trump’s announcement in April of sweeping global tariffs, the world’s two largest democracies seemed to be enjoying the friendship that its leaders had forged.

At a meeting with Mr. Trump at the White House in February, India’s prime minister, Narendra Modi, described India’s intention to become one of the world’s most advanced economies, with the United States as a partner.

“In the language of America, it’s ‘Make India Great Again’ — MIGA,” he said. “When America and India work together, this MAGA plus MIGA becomes a ‘mega partnership for prosperity.’” Mr. Trump smiled.