someday never comes.
It is a song about a son waiting for his dad to return from the war. The father tells his son that one day he will understand why he is going to war. I think that Fogerty is trying to paint the image that his son is quite young. With the boy being really young, Fogerty can say that the boy is not mature enough and when he gets older he will understand. The other may sound out in left field, but the complexity of why the US is fighting in Vietnam. Fogerty uses the boy so he doesn't hve to explain the reasons and motives behind another engagement in the Cold War.
The chorus is refering telling us that the soldiers aren't coming home. You (loved ones) better learn fast that there is going to be high casualties and don't expect your loved one to come home.
I think that the song is written for the Vietnam War, but it could also relate to the First and Second World War. The father in the first chorus is off to fight in the Great War (WWI). But his son grow up and has a child of his own. He gives a similar speech to his son, for he is leaving for war in September. WW2 starts on Sept 1, 1939 with the invasion of Poland by Nazi Germany. I am having trouble figuring out the reference to April. Hitler had taken over (anschluss) Austria and controlled Czechoslovakia by March 13-15, 1939. I don't know if Fogerty got his date wrong or something else happened...maybe with Japan and the war against China. I just thought that I would throw out the other idea that involved both world wars. I still think it is about Viet Nam.