It happened Tuesday afternoon in the area of Wilshire Boulevard and Vermont Avenue.
The victim, who only wanted to be identified as "Leo," said he was minding his own business, playing a game on his phone when he was approached by the unidentified suspect and punched in the face.
Video shows Leo on the ground, not moving. Another person is seen approaching to try and defuse the situation by engaging with the suspect, taking his attention away from Leo.
Leo said more Good Samaritans tried to intervene.
"He was sitting on the bench and I was standing up enjoying my game. I guess he just didn't like what I was doing. I had no music on, it was quiet, I was minding my own business, and he just came up and struck me violently," Leo said.
The veteran said he suffered a massive contusion on the back of his head and a cut on his nose from hitting the ground.
Leo said he has filed a police report with the LAPD.