The Valley-Westside War — Harry Turtledove

THE MISSION OF CROSSTIME TRAFFIC: TO TRADE WITH PARALLEL EARTHS WITHOUT EVER REVEALING THE SECRET OF CROSSTIME TRAVEL. IT'S A TOUGH JOB.

In a Los Angeles where nuclear war broke out back in 1967, survival is a matter of neighborhood versus neighborhood. When the Westsiders block Sepulveda Pass, the inhabitants of the San Fernando Valley are forced to fight back. With the help of some prewar machine guns, the Valley prevails, and their forces occupy the Westside.

In this brutal world, Liz and her family are undercover Crosstime Traffic agents living near the ruins of UCLA. Dan is a soldier in the occupying Valley army. Dan thinks Liz is the most impressive woman he's ever met. Liz thinks she'd better avoid Dan if she wants to protect the mission.

To complicate matters, when Dan catches Liz in the UCLA library, he fears she may be a spy for the Westsiders. After all, what reason could anyone have for reading about the old times, if not to figure out how to reconstruct old weapons systems?

Then a real spy for the Westside government-in-exile shows up at Liz's house....