Complex sentence: Majorian planned to reconquer Hispania and use it as the basis for the conquest of Africa: the richest province of the Western Empire, which provided for the very important grain supply to the city of Rome, was in fact under Vandal control.
Split into simpler sentences: Majorian planned to reconquer Hispania and use it as the base for the conquest of Africa. This rich province of the Western Empire, which provided for the very important grain supply to the city of Rome, was in fact under Vandal control.