Chicago Cubs Fan Explains Viral National Anthem Slap, Says He Apologized

A Chicago Cubs fan who went viral after smacking a younger man in the back of the head for sitting during the national anthem is breaking his silence ... and he tells us the video's missing a key ingredient.

Here's the deal ... over the weekend, an older gentleman in a Cubs jersey barked at a younger fan in front of him to get off his butt for the Star-Spangled Banner at Wrigley Field ... and when the person kept sitting, the Cubs fan smacked him in the head.

TMZ tracked down the Cubs fan -- Robert "Bob" Chavez -- and he tells us he lost his cool in the moment because he is a Vietnam veteran who had numerous buddies killed in the war.

Bob says he told the young man his veteran status and war stories when asking him to stand ... but the guy just kept sitting down, so Bob let loose a slap.

Bob says there's some important context missing from the viral video ... claiming both men made amends and apologized.

He says the younger man even bought him a cold beer ... and they explained to one another why they felt so strongly about the anthem and what they feel it represents.

Bob says security came over to kick him out ... but the younger man told them it was all good.

The Cubs confirmed Bob's narrative to TMZ, saying ...“The National Anthem is an important tradition at Chicago Cubs games. While we respect the rights of individuals to decide for themselves, we encourage every fan to stand, if they are able, and to remove their caps while the anthem is being played."

The club adds ... "At the same time, the safety and security of our fans is our top priority. Our security team spoke with the fan who had been struck. He indicated that the matter had been resolved, he did not want to have anyone ejected from the ballpark or any further action taken, and we respected his wishes.”