Rivalry and Temporary Detente
In regard to the Wisconsin/Michigan feud, my research traces it back to about 5:00 AM, May 10th, 1865. (General Lee had surrendered to General Grant on April 9.) It was at this point in time that a squad from the Fourth Michigan Cavalry commanded by Colonel Pritchard opened fire on members of Company M, First Wisconsin Cavalry commanded by Colonel H. Harnden. Before it was discovered that they were supposed to be on the same side, Wisconsin troops suffered several casualties, three severely, with two horses killed. Michigan had two fatalities and one wounded officer. They briefly set aside their differences and captured Jefferson Davis shortly afterward. (More details here.)
Those dadgum bushwhacking Michiganders! Even 141 years ago they would do anything to collect the $100,000 reward offered for the capture of the Confederate President. Small wonder that the rivalry continues to this day.
(H.T. Lance via his 'This Day in Wisconsin History' sidebar.)