“The Congress shall have Power To . . . provide for the common Defence and general Welfare of the United States.”
—U.S. Constitution, Article I, section 8, clause 1
“The Congress shall have 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 of Money 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”
—U.S. Constitution, Article I, section 8, clauses 11–16
The power to declare 'actual' war rests with Congress. Roosevelt's famous "Date of Infamy" speech was his speech to the joint houses requesting that they declare war on Japan after Pearl Harbor. The Gulf War was declared by Congress in 1991.
Now, that's not to say there isn't a way around that. Both Korea and Viet Nam were designated 'conflicts', not wars. And the '2nd Gulf not a War' was not declared by Congress but a result of Congress granting Dubya the authority to use military force in 2002's Authorization for Use of Military Force Against Iraq Resolution. None of those were 'technically' wars, but God knows they looked like wars to the poor men (and women) who got shipped off and the families waiting at home for them!
SCROTUS already renamed the DOD as "Dept of War", which it was previously named up until 1947. He's bound and determined to be a capital letters 'War President' and won't want to settle for being a small letters 'conflict president'. Will he decide to violate the Constitution based on the 2024 SCOTUS decision on presidential immunity and declare war? He would only go before Congress if he knew he had enough votes to pass a war declaration or to get an authorization similar to Dubya's. War declaration only requires a simple majority of those present. I think authorization for military action requires a simple majority of both houses.