Last night, President Obama announced that he has authorized airstrikes to respond to the humanitarian crisis in Iraq created by terrifying and brutal ISIS forces. U.S. strikes have apparently begun.

It is clear that President Obama believes he has the authority to act unilaterally, and members of Congress seem to either be acquiescing or else urging him to act even more aggressively. Senators McCain and Graham issued a statement last night calling on the president "to devise a comprehensive strategy to degrade ISIS." The senators warned that "the longer we wait to act, the worse this threat [from ISIS] will become, as recent events clearly show."

It is a mistake for Congress to cede authority to the president in this area. There is simply no reason to do so, and Congress's deference adds to the misconception that the president has legal authority to act alone. Unless there is an immediate threat to the United States that demands emergency action and allows no time for the president to consult with Congress, the Constitution does not permit the president to unilaterally order the use of military force. This is a position supported both by constitutional history and common sense. The framers of the Constitution rejected the then-prevailing British model, which assigned the king war power, and gave Congress the power to initiate war (outside of the emergency scenario where the president had authority to repel sudden attacks). In a system of checks and balances designed to prevent the concentration of power in any one branch, it is essential for the president to consult and cooperate with Congress on decisions about the use of military force whenever possible. History shows us that presidents make terrible mistakes when they act alone, and that they sometimes mislead Congress into rubberstamping their actions. Iraq itself should be a cautionary tale: President Bush mislead Congress into authorizing the war that began in 2003, while Senators McCain, Graham, and others incorrectly predicted that U.S. forces would be welcomed as liberators in an Iraq that would not fall prey to sectarian divisions.

Congress needs to assert itself by taking an active role in deciding what to do (or not do) about ISIS, rather than simply leaving it up to the president. Congress has shown that it is capable of playing this role. In the late summer of 2013, members of Congress correctly required the president to seek its approval before ordering military action in Syria. This was the right move both as a matter of constitutional law and policy: as the president waited for Congress to consider authorizing legislation, diplomatic developments obviated the claimed need for military action.

Why can the president act unilaterally in Iraq when he could not in Syria—what's different? Well, the United Nations Security Council has issued a statement condemning ISIS's brutality and calling on member states to help civilians suffering as a result of ISIS's actions. The Security Council did not endorse intervention in Syria. But even if the UN does ultimately call on member states to take military action (its statement does not expressly do this), that would not give President Obama authority to act. The Constitution, not the UN Charter, is the source of authority for military action, and the Constitution requires congressional approval outside of the emergency context.

President Obama has suggested that he can order military action because the Iraqi government has asked for help. Again, that is relevant only in the context of international law—of course the Iraqi government cannot take the place of Congress under the Constitution.

In theory, President Obama might point to legislation Congress passed in 2002 authorizing the 2003 war in Iraq. But that legislation was "purchased with false currency" and cannot be relied on as the basis for new military action.

In his address last night, President Obama promised that "as Commander-in-Chief, I will not allow the United States to be dragged into fighting another war in Iraq." But he cannot know where military action will lead and, under the Constitution, this decision is not his to make alone. ISIS is a terrible, brutal group, but it is not clear that U.S. action can stop them. Congress must insist on playing a central role in the decision-making process, instead of leaving it all up to the president.