Jingo by Terry Pratchett

As two armies march, Commander Vimes of Ankh-Morpork City Watch faces unpleasant foes who are out to get him... and that's just the people on his side. The enemy might be even worse.

Jingo, the 21st in Terry Pratchett's phenomenally successful Discworld series, makes the World Cup look like a friendly five-a-side.