What is the song played in Vietnam during South park episode Mexican staring frog of sri lanka?

The song played in South Park's "Mexican Staring Frog of Sri Lanka" episode during the scene in Vietnam is called "Happy Together" by The Turtles.

It's a popular 1967 song, and its use in the episode is meant to be ironic and humorous, highlighting the contrast between the cheerful music and the chaotic and violent situation in the war.