The Story of... 'Happy Xmas (War is Over)' by John Lennon
John Lennon and Yoko Ono wrote this in their New York City hotel room and recorded it during the evening of October 28 and into the morning of the 29th, 1971 at the Record Plant in New York. It was released in the US for Christmas, but didn't chart. The next year, it was released in the UK, where it did much better, charting at #4. Eventually, the song became a Christmas classic in America, but it took a while.
John and Yoko spent a lot of time in the late '60s and early '70s working to promote peace. In 1969, they put up billboards in major cities around the world that said, "War is over! (If you want it)." Two years later this slogan became the basis for this song when Lennon decided to make a Christmas record with an anti-war message. John also claimed another inspiration for writing the song: he said he was "sick of 'White Christmas.'"
The children's voices are the Harlem Community Choir, who were brought in to sing on this track. They are credited on the single along with Yoko and The Plastic Ono Band.
Source: SongFacts