War Powers Resolution. Obviously, there’s a law of which multiple presidents have used. Congress can change this law but there is a law that does give the POTUS this authority.

Nope, the War Powers Resolution gives the president broad authority to respond to an active attack on the United States (which makes sense). But it does not allow the President to unilaterally start an aggressive war against some random country without Congressional approval.

Not that we live in country where laws or the Constitution matter much right now. It's theoretically possible that some people might someday be prosecuted for breaking laws or violating people's Constitutional rights. But even there, I world expect that many of the law breakers will simply be pardoned.

What about the argument that Congress has always gone along with this in the past?

I mean it isn't quite that stark, but the last president that actually asked congress for and got a declaration of war was Roosevelt. The last president that asked for and got permission for the use of military force was George Bush (junior) after 9/11 (obv. he meant against the Taliban).

Which means all US conflicts are "based on" George Bush's approval for use of military force, about 1 per presidential term: military intervention in Lybia, the campaign against ISIS, campaign against Syria and Iraq militias/continuation against ISIS, and now Iran. Iran is a different scale I guess, but ...