We are now in the age of the imperial presidency. Presidents can do pretty much whatever they want, including starting major a war on their own. (Yes, LBJ and George W. Bush, I’m talking about you two.)
I don’t see this changing any time soon.
Neither LBJ nor Bush started any wars on their own. The Gulf of Tonkin resolution allowed Johnson to send combat troops to Vietnam. The Authorization for Use of Military Force of 2001 preceded operations in Afghanistan. The Authorization for Use of Military Force Against Iraq Resolution of 2002 gave Bush the green light to invade Iraq.