Terry Pratchett's Monstrous Regiment

War has come to Discworld. The homes and businesses throughout the duchy of Borogravia limp along, doing the best they can without their men, sent to fight their age-old enemy. Polly has taken over the lion's share of responsibility for the running of her family's humble inn, The Duchess. Her beloved brother Paul marched off to war almost a year ago, but it has been more than two months since his last letter home, and the news from the front is bad: the fighting has reached the border, supplies are dwindling, and the brave Borogravians are losing precious ground. So the resourceful Polly cuts her hair and joins the army as a young man named Oliver. As Polly closely guards her secret, she notices that her fellow recruits seem to be guarding secrets of their own.