Monstrous Regiment - Terry Pratchett

This is a stand-alone novel in the Discworld series, set in a country called Borogravia. In it a teenage girl, Polly, cuts her hair short and joins the army in an attempt to find out what has happened to her elder brother who enlisted the year before. The country is ruled by a Duchess that no one has seen for many years, and who would be extremely old by now. They have been attacking their neighbours and starting wars for as long as people can remember, and now it seems like it is catching up to them.