Ok dude, from one Kansan to another. If you haven't learned about this in school, well, I'm sure you don't live in Kansas.
So back in the 1860's Kansas was a free state, Missouri was white trash with bad roads (kinda like today heh ) Anyway, there were numerous border conflicts betwee Kansans and Missourians, many instigated by these two guys.
John Brown on the left and Quantrill on the right. Quantrill is the guy who burned Lawrence down, something I'm sure you think is cool Raptor, you lousy Wildcat :P
Heres my favorite pic JB He's the one who looks like Moses (btw That mural is 20 ft tall and in the KS Capitol building) Anyway, these guys triggered off the Infamous "Border Wars" Which triggered Order No. 11, the moving of all citizens of Jackson, CO MO to 3 desiganted towns controlled by the Union. Needless to say MO is still quite bitter to this day.
Fast forward, about "The oldesest football rivalry" thing. KU and MU were the first two schools west of the Mississippi to play each other regualrly each year in football, couple that with the above and bada-bing! There ya go I hope you're all enligtened and enriched.