Jingo

War is brewing on the Discworld.
An island has appeared from the ocean depths, right in the middle of the sea which separates the proud empires of Klatch and Ankh-Morpork. Of course, no one would dream of starting a war with the neighbours without a perfectly good reason . . . such as a 'strategic' piece of old rock, for instance.
But when a Klatchian Prince is almost assassinated, peace talks break down and violent nationalism begins to spread. Ankh-Morpork prepares to fight. Only thing is, they don't have an army. Or much in the way of weapons.
Commander Sam Vimes and the 'officially disbanded' City Watch get caught up in a deadly political game where the enemy appears to be on both sides and no one will listen to reason.
And if they don't stop this absurd war, no one will . . .
Jingo is the fourth book in the City Watch series, but you can read the Discworld novels in any order.