The whole event went down in Las Vegas on Thursday ... with Roman Reigns addressing the crowd and announcing he will face off against Dwayne Johnson in Philadelphia -- claiming it's his decision and there's nothing Cody can do about it.

Rock then showed up and hopped on the mic ... saying no matter what people think of the controversial matchup, they WILL respect it -- as Rock and Roman come from The Bloodline, the Samoan families with the richest history in professional wrestling.

Everything seemed set in stone as both guys shook hands -- but Cody wasn't having it ... taking the stage and calling the whole thing "bulls***," as he won the Royal Rumble and earned the right to enter the squared circle with Roman.

The guys traded nasty insults about each other's families ... and when Cody said Roman and Rock's grandparents would be ashamed of what they have become, The People's Champ lost it -- rocking Rhodes right in the mouth.

All sides needed to be separated and escorted off the stage ... but it didn't stop there, as a livid Rock confronted WWE chief content officer Triple H afterward, telling him to do something about Rhodes' disrespect -- or he'd handle it himself.

After it all died down, the WWE apparently sided with Rhodes in the whole ordeal ... revealing Cody would get his wish and take on Roman at WrestleMania 40.