WWII has gone down in American popular memory as "the Good War" (later criticized by authors like Studs Terkel, etc.).
But who coined this term? I don't mean the concept, which refers to the idea of "just war," but the particular term.
Was it already used in wartime media and propaganda? Or was it coined later as a contrast to the Cold War, Korean War, or Vietnam?
I've searched academic articles on the subject but they take the term for granted, never addressing its origins.
From what I can see, the term "The Good War" when used for WWII was coined by Studs Terkel. If you look at the Google ngrams as Mark C Wallace suggests, you see two spikes, one small one starting in late sixties, and one large one starting in the late eighties.
In 1965, Marian Maury published The good war: The UN's world-wide fight against poverty, disease and ignorance. This is not about WWII.
In 1984, Studs Terkel published "The Good War": An Oral History of World War II.
In this interview, Terkel says "In the book about World War II, I call the good war. but you notice there quotation marks around the phrase". This is not concrete, but it does sound like he feels he originated the phrase. Here's another interview clip that talks a lot about the feelings about the war. There's no implication that "The Good War" as a phrase is other than Terkel's "ironic" title.
It seems as if most usages of the term "The Good War" originated from one of those two books, so I think it's safe to say that the term was coined by Terkel himself when used for WWII. It is hard to prove a negative, but I don't think it was ever used directly before this, or as propaganda. It looks to me that it has always been used after this with a hint of irony.