Ok, when is the last time we had a formel declaration of war?
In order for the Congress to declare a state of War there has to be a nation to declare war against. We are not at war with a nation state, we are at War with a bunch of people called terrorists.
September 14, 2001: Authorization for Use of Military Force Against Terrorists.
Under that, Congress authorized us to wage war against al-Qaeda and associated supporting jihadist organizations. ISIS qualifies under that, in that they have supported terrorist acts on US soil. Therefore, it is legal for US military to hit ISIS targets.
The government of Syria, as far as I know, has NOT given support to terrorist acts against the US.
Back in 2013, Obama asked Congress for authorization to go against the government of Syria, indicating that even HE understood that he needed Congressional approval. The bill never went to a vote.