The United States has spent decades trying to blunt China’s strategic rise. Yet in the Middle East, sanctions and the war against Iran are now dismantling one of the few assets built to constrain Beijing’s influence in the region. The Iranian port of Chabahar has received strong backing from India, which hoped it would gain both a trade corridor into Afghanistan and Central Asia that bypassed Pakistan and a strategic counterweight to the Chinese-operated Pakistani port of Gwadar. Both Washington...