When and where did this originate?
It’s called the Battlefield Cross, and probably originated during the American Civil War.
I wasn’t aware that soldiers in the Civil War wore military helmets.
No but they did have hats.
Is it not dumb to leave rifles behind on a battlefield?
They are just temporary markers. Originally it seems they were just used to mark where a solider fell so that the body could be retrieved and buried elsewhere. They then would have been removed.
From the first link, at present: