The platoon of Chinese soldiers slipped across the boundary into India in the middle of the night, according to Indian officials. They were ferried across the bitterly cold moonscape in Chinese army vehicles, then got out to traverse a dry creek bed with a helicopter hovering overhead for protection. They finally reached their destination and pitched a tent in the barren Depsang Valley in the Ladakh region, a symbolic claim of sovereignty deep inside Indian-held territory. So stealthy was the operation that India did not discover the incursion until a day later, Indian officials said. China denies any incursion, but...