Under Article II, section 2, the President is invested with the power as "Commander in Chief of the Army and Navy of the United States".
On the other hand, under Article 1, section 8, Congress has the power to declare war.
This creates an interesting question:
Does the President have command authority over the Armed Forces before a Congressional declaration or after? and if so why?
Looking back to the days that the Constitution was drafted, if Congress was not in session, it might take days or weeks to assemble Congress to declare war which creates an issue in the case of a sneak attack. This would require that the President be able to act without Congress' war declaration.
In today's environment (and arguably in the last 50+ years), this is not the case. Congress can convene and hold a declaration of war vote within hours.
As to why: A war cannot be commanded by Congressional committee. There must be a single command authority and that power has been invested in the President.
In an attempt to reconcile this issue, Congress passed the War Powers Act in 1973. This limited the President's power to as C in C of the Armed Forces and put it more under the control of Congress. It's passage was not without controversy. Congress passed it, the President vetoed it and Congress overrode the veto.
There is a significant Constitutional issue at play here (who has power over the military?) and neither side is really willing to submit this to Supreme Court adjudication. One side or both run the risk being neutered when it comes to the use of military depending upon how the Supreme Court might rule so they will sit on their hands until they can no longer do so.