Not too explicit, but the cover of Batman 12 (Aug/Sept 42) is a war cover. This is also the first time an issue 12 is billed as an “anniversary issue.”
The first story in this issue, Brothers in Crime, by Don Cameron and Jerry Robinson, is also the first tale to really show the Trophy Room, in the days when it was independent of the Batcave.
The Trophy Room is clearly located within Wayne Manor in this story. We some remnants of earlier cases, including the Prophetic Crimes. Notably lacking are the giant penny and mechanical dinosaur, but the stories that introduced those items are in the future. This story is largely told in flashback, as Batman and Robin inspect a chainmail vest, and recall the events that lead to its inclusion in the Trophy Room.
The vest passes from member to member of the Rafferty family, three brothers who all walk on the wrong side of the law. They wear the vest in order to protect themselves, even though it winds up causing the deaths of each of them, in pretty horrific ways.
The vest itself is never seen again, but the Trophy Room would become an essential element of the Batman mythos.
Finger, Kane and Robinson bring back the Joker, as well as Lee Benson, in this story, which sees the Joker plan a crime spree based on the literal interpretation of slang terms.
Lee Benson, the District Attorney from the previous issue, gets a cameo in this story.
He gets “framed” by the Joker, one of many city officials who suffer through the Joker’s pranks. As we never see Benson again, one must assume that he lost his position, after his compromising behaviour in the previous issue, and the public humiliation in this one.
Honestly, the Joker’s plot in this tale is about the least important thing in it. Aside from Benson’s cameo, this story also has the first view of the famous cutaway that would get expanded over the years. There is nothing much cave-y about this Batcave (and the phrase is not used). It is basically just a big underground garage in this story. The Trophy Room is not part of it, but the existence of an elevator to the Manor (just called the Wayne House) gets established.
There is an enjoyable climax to the tale, a fight between Batman and the Joker on a blimp, which of course allows the Joker to plummet to his apparent desk.