Passage: The Wolverines find a downed American pilot, Lt. Col. Andrew Tanner, who informs them of the current state of the war: several American cities, including Washington, D.C., were destroyed by nuclear strikes; the Strategic Air Command was crippled by Cuban saboteurs; and paratroopers were dropped from fake commercial airliners to seize key positions in preparation for subsequent assaults via Mexico and Alaska. The middle third of the U.S. has been taken over, but American counterattacks have halted Soviet advances along the Rocky Mountains and the Mississippi River and the lines have stabilized. The only remaining U.S. allies, the UK and People's Republic of China, are militarily crippled. Concerned about nuclear fallout, both sides refrain from the further use of nuclear weapons.

Question: Why do the factions refrain from using nuclear weapons?
nuclear fallout