New Delhi, Feb. 20 -- A BIT OF sabre-rattling can be useful in foreign policy. Too much of it, though, and the sabre is liable to end up pointing back at you. That is where America finds itself after a two-month standoff with Iran. Donald Trump, the president, has ordered an enormous military build-up in the Middle East-by some measures the biggest in two decades. Yet he has seemed unwilling to order a risky strike. Nor have his threats yet compelled Iran to negotiate a deal that would prevent one. Now he is left with a difficult choice: order an attack anyway, or beat an embarrassing retreat.

This crisis, the latest of many in America's tortured history with the Islamic Republic, began in late December when a wave of protests erupted in...