Foreign military strikes on Syria would be illegal under international law unless authorized by the Security Council. With the singular exception of a country’s inherent right to self defense, the Security Council — and only the Security Council — has the legal authority to permit a country’s sovereignty from being violated in this way. Of course, being illegal does not necessarily make it wrong. There are compelling moral and practical arguments to justify military strikes against the Syrian government; chemical weapons themselves are illegal and their use must be punished and deterred. Also, these are inherently indiscriminate weapons and if their use becomes routinized in conflict then war will become much, much more brutal. The administration is currently weighing legal justifications for circumventing the council, including having Jordan and Turkey invoke an expansive definition of the right to self defense; or even appealing to the Geneva Protocols or the Chemical Weapons Convention. Those may be convenient crutches, they do not make the intervention legal. Only the Security Council can do that. Circumventing the Security Council by invoking spurious legal authorities may be the expedient thing to do, but it is not without consequences. When you dilute the authority of the Security Council, you make it easier for countries to launch wars. The Kosovo campaign stopped ethnic cleansing, but it also set a precedent that was followed by George W Bush to mount an equally illegal (and much more foolhardy) invasion and occupation of Iraq. If President Obama leads the charge to intervene in Syria without Security Council approval, he would grievously harm a basic tenant of international law. If he does, might other countries feel less constrained by the Security Council? Might President Ted Cruz cite Obama’s circumvention of the council as justification for a similarly illegal military campaign? Strikes against Syria may be morally justified, but they will be illegal. Diluting the authority of the Security Council comes with a cost. We must be cognizant that there will be a price to pay down the line for undermining international prohibitions on who gets to decide when to go to war.