NEW DELHI — Longtime adversaries India and China, concerned by President Donald Trump’s disorienting early moves in trade and diplomacy, are testing a fragile thaw in relations.

For most of the past century, a dispute over their 2,000-mile Himalayan border has kept the world’s two largest countries deadlocked in distrust. But as Trump disrupts decades-old alliances, upends regional balances of power, and taxes imports from friend and foe alike, India has become increasingly receptive to overtures from China. Those overtures have grown in intensity as Trump has ratcheted up tariffs on goods from China.