Battle of Crumhorn
The Battle of Crumhorn was the first battle in the universe where the enemy were persuaded to sell their weapons. In Gaspode's opinion, they "shoulda put up a statchoo to the accountants". Instead, the commemorative arch has over 90 statues of famous soldiers. The actual practical design was handled by Bergholt Stuttley Johnson, resulting in the only easily-stolen landmark that exists. The arch is just one of a few monuments kept in a box by Mr Scant.
As the arch was commissioned to commemorate the battle by the ruler of Ankh-Morpork we can assume that Ankh-Morpork was both involved in the battle and the victor (as defeats are rarely celebrated), but who they were fighting remains unknown.