It was most powerful in Southern Germany.

This was countered immediately by the foundation under Maximilian, Duke of Bavaria, of a Catholic League.

The friction, which was now becoming acute, went on increasing till the actual outbreak of the Thirty Years' War in 1618.

The signal for the latter was given by the Bohemian revolution in the spring of that year.
The Thirty Years' War, as it is termed, which was really a series of wars, naturally falls into five distinct periods, each representing in many respects a separate war in itself.

The first two years of the war (1618-20) is occupied with the Bohemian revolt against the attempt of the Emperor to force Catholicism upon the Bohemian people and with its immediate consequences.