The definition of Thirty Years' War in the dictionary is a major conflict involving principally Austria, Denmark, France, Holland, the German states, Spain, and Sweden, that devastated central Europe, esp large areas of Germany. It began as a war between Protestants and Catholics but was gradually transformed into a struggle to determine whether the German emperor could assert more than nominal authority over his princely vassals. The Peace of Westphalia gave the German states their sovereignty and the right of religious toleration and confirmed French ascendancy.