James Madison, the principal drafter of the Constitution, wrote that the history of mankind showed that the executive branch is “most interested in war, & most prone to it.”

For this reason, he noted that the Constitution, “with studied care, vested the question of war” in the legislature. Once initiated, the power to carry out military action does flow to the president as commander in chief. And the president always has the power to defend the nation from imminent attack. But even when the president acts unilaterally in response to an attack, that action must be brief and limited to addressing a specific threat. Any action beyond that scope requires an authorization by Congress.