While reasonable people can debate the wisdom of killing Iranian Maj. Gen. Qasem Soleimani, there is little doubt that President Trump acted lawfully—under both domestic and international law—in ordering his death. The president has the constitutional authority to take military actions, short of declaring war, that he and his advisers deem necessary to protect American citizens. This authority is extremely broad, especially when the actions must, by their nature, be kept secret from the intended target.

Congress has the sole...