The Constitution says That Congress gets to
DECIDE when, how and if we go to war.
The president only gets to decide what it is that the various services get to do once Congress declares war.
Article I, Section 8:
The Congress shall have the power....
To declare War, grant Letters of Marque and Reprisal and make Rules concerning Captures on Land and Water;
To raise and support Armies, but no Appropriation to that Use shall be for a longer Term than two years;
To provide and maintain a Navy;
To make Rules for the government and Regulation of the land and naval Forces;
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress.
....
Article II, Section 2:
The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States;....
Time to take the toys away and put him in the corner for a very long time-out...