As commander in chief the president has a widely recognized and long established power to take emergency action to deal with military and security threats. And the president also has the power to deploy the US military on non-combat operations for humanitarian purposes—for example, when the US Navy responded to the Indian Ocean tsunami. But acts of war against an enemy that does not directly threaten the United States of America or its treaty allies must pass a tougher test.
If the president really can launch discretionary military attacks on humanitarian grounds around the world at will, we have an elected dictatorship, not a system of limited powers. Is the President of the United States to be the judge, jury and enforcer of international law even when nothing in either US or international law gives him these powers?
As a practical matter, one can see circumstances (a fast moving wave of genocidal violence while Congress is out of session, for example) in which a president could responsibly substitute consultation with Congressional leaders for a full and formal vote. But what President Obama wants in Syria is a retaliatory strike. He is not intervening rapidly to stop a wave of chemical attacks. He is acting at leisure, with reviews of evidence, international consultation, reports from observers. It seems to make little difference whether he acts on it today or tomorrow or next week. There is, evidently, no practical reason for failing to consult Congress; the President himself has chosen to postpone any military action until Congress acts on the matter. In such a case it seems very hard to create a sound constitutional argument justifying presidential action if Congress rejects his proposal.