Until the 20th century, historians considered it a continuation of the German religious struggle initiated by the Reformation and ended by the 1555 Peace of Augsburg. This divided the Empire into Lutheran and Catholic states, but over the next 50 years the expansion of Protestantism beyond these boundaries gradually destabilised Imperial authority. While a significant factor in the war that followed, it is generally agreed its scope and extent was driven by the contest for European dominance between Habsburgs in Austria and Spain, and the French House of Bourbon.