What is the main conflict in Swan Song by Robert McCammon?

There are quite a few conflicts in the story. The main conflict involves a post-apocalyptic United States of America and its inhabitants. The USA and the U.S.S.R have destroyed each other with nuclear weapons. On their way out of desolate Manhattan, Sister and Artie find a magical glass ring that allows them to dream walk. They also encounter an evil character, later known as Friend, who pursues them across the country.