I remember hearing this one over the years and regarded this as a very pleasant oldie-but-goodie, not knowing much about the story behind it. Jiros （ジローズ）started life as an amateur band headed by Jiro Sugita（杉田二郎）, while he was a student at Ritsumeikan University in Kyoto in 1967. The name was chosen since Sugita and the two other members had similar first names. However, this version of the band broke up a year later.
Then in 1970, Sugita started the 2nd version of Jiros, this time with Jiro Morishita（森下次郎） partnering up with him, with their 2nd of 6 singles, "Senso wo Shiranai Kodomo Tachi"(Children Who Don't Know War) becoming their biggest hit. Released in February 1971, it was composed by Sugita and written by musician Osamu Kitayama（北山修） who also had his own big hit during that year. According to the Wikipedia entry on the song, with the Vietnam War raging and US troops stationed in Japan at the time as citizens were still dealing with the aftermath of World War II, the song was seen as a mild and happy-go-lucky anti-war song, although Jiros never reflected any direct association with the peace movement.
And going through the lyrics, I don't see any particular anti-war sentiment, although Jiros sing about the then-young generation being as happy as sheep in their long hair, growing up in a time of no war and humming peace songs. If anything, I think the two were pushing (ever so slightly) more for hedonism. And in the same Wiki article, lyricist Kitayama later considered the song "....a puerile effort to satirize, and rebel against, the contempt felt by older generations who experienced WWII for younger people born in the postwar period--who were denounced for weak-mindedness and lack of self-discipline...". Uh, ouch!
Yeah, I think the song is so pleasantly sung that those same older generations that Kitayama referred to probably happily clapped along to it. I didn't especially get any feeling of lyrically sharp slashes slicing through the fabric of society any more than a plastic knife would carve through a frozen block of butter. But that's me. In any case, the song was a pretty decent hit with it peaking at No. 11 on Oricon, and winning two Japan Music Awards for Best Newcomer and Best Lyrics.