 Consequently, the assumptions and theories developed by historians are not definitive, and debate still rages on many issues, such as the breeding or size of the horse, and a number of sources must be consulted in order to understand the breadth of the subject.During the decline of the Roman Empire and the Early Middle Ages, much of the quality breeding stock developed during the classical period was lost due to uncontrolled breeding and had to be built up again over the following centuries.In the west, this may have been due in part to the reliance of the British and Scandinavians on infantry-based warfare, where horses were only used for riding and pursuit.