Article I, Section 8 of The US Constitution states in part, "Congress shall have The Power to...To declare War, grant Letters of Marque and Reprisal and make Rules concerning Captures on Land and Water."
These delegated and enumerated powers are pretty conclusive on which body has the power to declare war. It is not the President. In addition The POTUS does not become Commander in Chief until such time as there has been a formal declaration of War rendered by the Congress. War powers was a gift from Congress to give cover to a president in order that he may continue unlawful wars like the one we are being consumed by presently. Conversely, the Congress can blame the POTUS, and I suspect this is more case, when the War is going bad.
It should be obvious that this inbred major party system (Repub/Dem) is the root of the problem that plagues our country. It is time to clean these guys out en mass. We need at least one third of both Houses of Congress to consist of Independents. Nothing else can counter the partisan collusion that continually favors one party or the other. Vote for anybody that is not a member of the two greatest organized crime syndicates in the World, a Congress controlled by either Democrats or Republicans. Being elected to Congress was never intended to be a career spanning decades. Getting rid of these bought and paid for life long members of Congress is the only thing that will save the US.