What is the "war" called in "By the Waters of Babylon"?
This question can be answered with a close reading of the text, and can be found pretty quickly in the story, at the end of the very first paragraph. In John's culture, their people are so far removed from the war that it itself has become a thing of legend, and has been generalized over and over, until it isn't even remembered as a war. His people don't quite understand that the Place of the Gods was an actual human city that was wiped out by other humans. They don't grasp the amount of cruelty and violence that mankind would have had to have had for each other in order for that to occur. It has been so very long since the incident, and knowledge so limited that everything is fable, legend, and a watered-down version of the truth. So, instead of referring to it as a war, which is what it was, they call it "the Great Burning." The story states, at the end of the first paragraph,
"It is forbidden to cross the great river and look upon the place that was the Place of the Gods ...it is there that there are the ashes of the Great Burning. These things are forbidden -- they have been forbidden since the beginning of time."
Their knowledge is so limited that they feel that going to the Place of the Gods has been forbidden "since the beginning of time," which is inaccurate. But, the amount of time that has passed has been so great that it has become so in their culture. I hope that those thoughts helped a bit; good luck!