I nominate the bewildering changes of allegiance during the War of the League of Cambrai.
Spain, the Holy Roman Empire, Switzerland, Milan, Florence, England and Scotland were all drawn into it, but the heart of the fighting was in North Italy where the alliances unfolded as follows:
1508: Papal States and France ally against Venice, which had seized papal territory from Cesare Borgia's forces.
1510: Papal States gets alarmed by the French successes and allies with Venice against France.
1513: By this time, Spain, the Holy Roman Empire and England have also entered the war against France, their common enemy. Of these, the Holy Roman Empire is trying to take territory from Venice. With no-one else to turn to, France and Venice join forces against the Papal States and its allies.
(When the war finally ended, the territorial map was about the same as when the war started.)