Jingo: (Discworld Novel 21) - Terry Pratchett

A new land has surfaced and so have old feuds. And as two armies march, Commander Vimes of Ankh-Morpork City Watch has got just a few hours to deal with a crime so big that there's no law against it. It's called "war." He's facing unpleasant foes who are out to get him . . .that's just the people on his side. The enemy might even be worse. And his pocket Dis-organizer says he's got "Die" under "Things to do today.