Six months into a conflict neither side has found a way to end, the Persian Gulf has become the arena for a dangerous test of wills between Washington and Tehran. Iran's parliament speaker has warned that American strikes will be met with responses faster, heavier, and more painful than before — words that arrive as a closed Strait of Hormuz reshapes global energy markets and presses on the lives of ordinary people far from the fighting. History has seen great powers circle this waterway before, but rarely with so much at stake and so little diplomatic ground left to stand on.