India, July 14 -- The Strait of Hormuz, a narrow, 21-mile-wide channel of water through which a fifth of the world's traded oil and natural gas normally passes, has become the defining front in the war between the United States and Iran, a battle over who controls the waterway that has closed it to most commercial traffic, sent crude oil prices surging, and brought Washington and Tehran to the edge of a full rupture just weeks after they signed what was billed as a peace deal.

US President Trump declared Monday that the United States would henceforth be known as the "Guardian of the Hormuz Strait," reinstating the naval blockade of Iranian ports lifted under last month's ceasefire and announcing a 20 per cent toll on all cargo transiting...