explain the meaning of the phrase alas Babylon

"Alas, Babylon" is the quote that serves as the title of the novel. Years before, Randy and Mark agreed that should Mark get wind of an approaching nuclear attack, he would warn Randy in some way by passing along the words "Alas, Babylon," because it would be risky to say the truth over telephone or telegram. This quote has biblical origins; in the Bible, Babylon was a city of sin destroyed by God. It is fitting, because many cities in the U.S. are similarly destroyed, but by nuclear attacks.