[Platapus]

Under the War Powers Act of 1973, the POTUS can only introduce US Armed Forces into hostilities under one of only three conditions

1. A declaration of war by Congress
2. Specific statutory authorization (this is what President Obama is asking for)
3. "a national emergency created by attack upon the United States, its territories, or possessions, or its armed forces." That is a direct quote from the War Powers Act of 1973.

It would be difficult to consider what is happening in Syria as applying to item 3.

So no the president can't legally just decide to send US Military into hostilities. That's the whole purpose of the War Powers Act, and the reason no President since 1973 has recognized the constitutionality of the War Powers Act.

If in the instance of item 3, the President must provide specific information as soon as possible. The exact words are "The President in every possible instance shall consult with Congress before introducing United States Armed Forces into hostilities or into situation where imminent involvement in hostilities is clearly indicated by the circumstances, and after such introduction shall consult regularly with Congress until United States Armed Forces are no longer engaged in hostilities or have been removed from such situations. "

If after 60 days from either the delivery of the first report or the deployment of US Armed Forces, the Congress has not either declared war nor issued a statutory authority, the President must recall all US Armed Forces. There is an option to extend this an additional 30 days for the purpose of withdrawal logistics.

So no the president can't just decide to send in the troops and yes the president needs congressional approval if he can make a legal decision to send in the troops.

I think the War Powers Act is a necessary check on the powers of the Executive and Commander in Chief.