BEIT SHEMESH, Israel — Even before U.S. and Israeli warplanes struck Iranian targets, officials in Washington and Jerusalem were bracing for retaliation. They had gamed out missile salvos, cyberattacks, proxy strikes on U.S. bases and Israeli cities. What few seemed prepared for was the speed — and the geographic sprawl — of the blowback: some 300 million civilians, across more than a dozen nations, suddenly under the umbrella of an escalating war.