The Battle of White Mountain, November 8, 1620 (Bílá hora is the name of White Mountain in Czech) was an early battle in the Thirty Years' War in which an army of 20,000 Bohemians and mercenaries under Christian of Anhalt were routed by 25,000 men of the combined armies of Ferdinand II, Holy Roman Emperor under Karel Bonaventura Buquoy (as he is known in Czech; Charles in his native French) and of the Catholic League under Johann Tserclaes, Count of Tilly at Bílá Hora, near Prague (now part of the city). The battle marked the end of the Bohemian period of the Thirty Years' War.
Tilly observed the enemy position and sent his well-trained men over a small bridge crossing the brook. In just two hours of heavy fighting, they smashed through the center of the enemy line. This decided the battle.
With the Bohemian army destroyed, Tilly entered Prague and the revolt broke down. King Frederick with his wife Elizabeth fled the country (hence his nickname the Winter King), and many citizens welcomed the restoration of Catholicism. Forty-seven noble leaders of the insurrection were tried, and twenty-seven were executed on what is called "the Day of Blood" by Protestants at Prague's Old Town Square. Today, 27 crosses have been inlayed in the cobblestone as a tribute to those victims.
In 1621, the Emperor ordered all Calvinists and other non-Lutherans to leave the realm in 3 days. Next year, he also ordered all Lutherans (who primarily had not been involved in the revolt) to convert or leave the country. By 1627, Archbishop Harrach of Prague and Jaroslav Borzita of Martinice set out to peacefully convert the heretics; most Bohemians converted, but a significant Protestant minority remained. Spanish troops, seeking to encircle their rebellious Dutch provinces, seized the Palatinate. With Protestantism threatening to be overrun in Germany, Denmark entered the struggle.