In most accounts of the the Spanish Armada, Alonso Perez du Guzman is referred to by his aristocratic title, 6th Duke of Medina Sidonia, or simply as Medina Sidonia. Although Medina Sidonia had much experience in Phillip II's military campaigns on land and was considered perhaps Spain's best military administrator, Medina Sidonia did not have much direct experience in naval warfare. In fact, Phillip II originally intended command of the Armada for Alvaro de Bazan, the Marques de Santa Cruz, Spain's most formidable naval commander, but he died as the ships, supplies, and men of the Armada were being assembled by Medina Sidonia. Because Medina Sidonia had been so intimately involved in the formation of the Armada, and knew both ships and their commanders fairly well, he was the natural commander to replace de Bazan. Medina Sidonia expressed his concerns about command to Phillip II, but Phillip considered him to be the next best choice of commander. In any case, the Armada, even though it was a formidable naval force, was supposed to weaken the English navy, pick up Spanish land forces in Flanders, and put them ashore in England. The Armada's goal was not to conquer England but to render England defenseless so that Spain could conquer England with troops.
In the event, Medina Sidonia's lack of naval experience was not among the primary causes of the Armada's failure, which was the result of several factors including weather, England's ship technology, shipboard gun technology, poor communication with Sain's ground forces in Flanders, and England's resolve to stop a potential invasion.