The United Kingdom is on a trajectory toward slow-motion collapse. British Prime Minister Kier Starmer’s initial reaction to the outbreak of the war against Iran was schizophrenic. He seemed wholly unprepared for a scenario in which Donald Trump would actually use the forces he had spent the previous two months flooding into the region, confounding the former Royal Navy officers and opposition lawmakers who somehow understood that Trump’s ultimatums weren’t just talk. At first, Starmer was appalled, and he blocked the Pentagon from using U.K. bases to launch sorties against Iranian targets. But the impracticality of that protest sapped his...