President Trump has staked enormous political capital on being the one to end the war in Ukraine, even asserting that he could do so “in 24 hours.” In perhaps his biggest gamble yet to achieve that goal, he pledged on Wednesday to punish India with tariffs of 50 percent for buying Russian oil.

At stake is the relationship between the United States and an increasingly important strategic partner in Asia. India, the world’s most populous democracy, and the United States, its most powerful one, have an unusual relationship. They are friendly but not close, brought together by mutual interests and shared values, especially in recent decades.

On Wednesday, Mr. Trump seemed ready to ditch that relationship. He doubled already hefty tariffs on Indian exports to the United States for its steadfast refusal to stop buying oil from Russia, in an effort to pressure Russia to end the war. Mr. Trump has accused India of helping Russia finance its war on Ukraine through oil purchases; India has said it needs cheap oil to meet the energy needs of its fast-growing economy.

India called the additional tariffs “unfair, unjustified and unreasonable,” pointing out that it was being punished for doing something — buying Russian oil at a discounted price — that other nations have done, although it didn’t mention names. China is the largest buyer of Russian oil, and Turkey has also deepened its energy links with Russia since the start of the war in Ukraine, without incurring similar penalties.

Analysts said Mr. Trump’s pressure tactics could damage the longstanding ties between India and the United States.

“We are better off together than apart,” said Atul Keshap, a retired U.S. diplomat and president of the U.S.-India Business Council. “The partnership forged by our elected leaders over the past 25 years is worth preserving, and has achieved considerable mutual prosperity and advanced our shared strategic interests.”