Monstrous regiment / Pratchett, Terry

Polly Perks, fearing that the national deity's decree against female ownership of a business is going to cause her to lose the family's inn, disguises herself as a man in order to join the military and find her brother who she fears has been killed in Borogravia's latest war.

The arrogantly fundamentalist duchy of Borogravia is so militarily aggressive that it can't afford to be too picky about who it recruits for its army.