If a player ever moves into a region, which starts a war with an ally, and doesn’t offer peace, all allied regions they share are in jeopardy from this war. The war’s winner will claim control of all the regions they share with the losing player. The player who lost must retreat all their meeples in shared regions, paying the cost of retreat for each meeple, or remove them from play. The order in which these lost regions are resolved is chosen by the player who lost.
If breaking an alliance would result in a player losing the last of their meeples, and that player cannot retreat a single meeple, then that player may keep 1 meeple for free and must place that meeple in the nearest unoccupied region, even if that means placing it in a region on a different territory card.