Remember Koom Valley!

So, at the moment I'm re-reading Thief of Time by Terry Pratchett. And just somewhere in there, easy to overlook, I found this one little annotation.
It's about history and about the battle of Koom Valley. The Discworld Wiki says about that battle:

“Remember Koom Valley!” Hundreds of years ago, the trolls and the dwarfs met in a battle at Koom Valley. It was a remarkable battle in that each side ambushed the other and, of course, in being the historical event symbolizing the enmity between trolls and dwarfs. Nowadays, dwarfs celebrate the Battle of Koom Valley Day, on the same day that trolls celebrate Troll New Year. Sometimes, in a city where both dwarfs and trolls live (e.g. Ankh-Morpork), the two groups are careful enough to plan their celebratory marches so that they are coincidentally on the same street (see Men at Arms).

And here's the annotation (Thief of Time, p. 215):

Every society needs a cry like that, but only in a very few do they come out with the complete, unvarnished version, which is 'Remember-the-Atrocity-Committed-Against-Us-Last-Time-That-Will-Excuse-the-Atrocity-That-We're-About-to-Commit-Today! And So On! Hurrah!'

By the way, the book was released in 2001. I couldn't find any exact date, but Terry Pratchett must have been done writing before September of that year.