The Peanuts gang heads off to summer camp for the first time in 1969's It Was a Short Summer, Charlie Brown. At the time, the specials were airing on CBS on a seemingly annual basis, and the pace would pick up during the 70's.
Charlie (Peter Robbins, ex-Blondie) and Peppermint Patty captain the teams in a lopsided battle of the sexes. The boys reflect Charlie's personality, as they can't seem to get out of their own way, while the girls prove to be more organized and superior in every event. Imagine having these kids in a Battle of the Network Stars-type competition.
The complete episode is not available. Instead, here's a compilation of clips, culminating in the infamous wrist wrestling match between Lucy and the Masked Marvel (Snoopy, of course).
Some wiseguy on Wikipedia made an editing goof, trying to claim that Chuck Jones and DePatie-Freleng had something to do with this when they clearly didn't. Probably transposed the information from The Cat in the Hat.