This one's pretty simple; I was invited to submit a story to an upcoming anthology of Riverworld stories, based on Philip Jose Farmer's series about a world where dead people from throughout human history wake up alive again on the banks of a mysterious river. It occurred to me that in Riverworld the nature of war would be different, because the nature of death is different.
So I thought about great warriors, and decided it would be interesting to see what George Patton and Germanicus Caesar might do together.
That's it, basically. I wrote the story, they bought it, and there it is. The anthology is Quest to Riverworld.
That's it; here's your list of handy exits: