To reproduce:

0. set autosave delay to 0 (each turn)

1. wait for AI to attack your city or an army (and lose that battle for more clear effect)

2. at your next turn load latest autosave

3. you'll find our yourself at the same turn but without preceding AI actions (like all AI and monsters skipped their turns) -- e.g. your city/army alive

Didn't tried it but it seems with using this trick every turn from the start you can find out and kill all AI sovereigns before they even found capital