What is the Great Burning and Destruction that is talked about in "By the Waters of Babylon"?

We actually "see" for ourselves the "Great Burning" that
destroyed our civilisation in this dystopian masterpiece during the vision that John has
whilst he is sleeping in one of the towers in the city. Note what John says about what
he sees:

When
gods war with gods, they use weapons we do not know. It was fire falling out of the sky
and a mist that poisoned. It was the time of the Great Burning and the Destruction. They
ran about like ants in the streets of their city - poor gods, poor
gods!

We need to remember
that because this story is told from the first person perspective, we see everything
through John's eyes. He does not know about sophisticated military weapons, so he only
describes what he sees. We can infer from his description however that "the Great
Burning" and "the Destruction" were caused by advanced nuclear and chemical weaponry
that brought fire down upon the world and poisonous gas.