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.
Yes, resolutions are what gave LBJ and George W. Bush cover to start their wars. But that’s entirely unconstitutional. You wan to start a war, you go before Congress and ask for a declaration of war.
Congress gets a big share of the blame here. Instead of following the Constitution, they avoid taking responsibility. So they pass a resolution, and then let the president-king call all the shots.