In Tehran, the capital of Iran, security forces opened fire at protesters from the roof of a police station. In Karaj, they fired live rounds into a march, shooting one person in the head. In Isfahan, young men barricaded themselves in an alley as gunfire and explosions rang out. Scattered protests had percolated since late December, starting with a strike in Tehran’s bazaar and fueled by a plunging economy. But by early January, Iranians had revolted en masse, and the security forces began to crack down with deadly force. It was not just the protests unnerving the regime. President Trump...