Great Stirrup Controversy
The Great Stirrup Controversy is the academic debate about the Stirrup Thesis, the theory that feudalism in Europe developed largely as a result of the introduction of the stirrup to cavalry in the 8th century AD. It relates to the hypothesis suggested by Lynn Townsend White Jr. in his 1962 book, Medieval Technology and Social Change.
Source: Wikipedia — Great Stirrup Controversy (CC BY-SA 4.0)