The US and its allies are considering a military intervention in Syria in response to the Assad regime’s use of chemical weapons against civilians. If President Obama decides to go in, it will likely be without congressional authorization. Such a step would create serious constitutional problems similar to those arising from the intervention in Libya in 2011.
Article I of the Constitution reserves the power to declare war to Congress alone. Thus, any military action large enough to constitute a war requires congressional authorization. The president can, of course, defend against an actual or imminent enemy attack without waiting for Congress. In that scenario, a state of war would already exist independent of any US action. But the Assad regime has not attacked the US and does not seem likely to do so in the near future. Everything I said in this 2011 National Review symposium essay on the Libya War applies with equal force to a possible strike on Syria.