The Battle of White Mountain on 8 November 1620 between the Bohemian Estates and the Holy Roman Empire, Spain and the Catholic League was an early battle in the Thirty Years' War in which an army of 30,000 Bohemians and mercenaries under Christian of Anhalt were routed by 27,000 men of the combined armies of Ferdinand II, Holy Roman Emperor under Charles Bonaventure de Longueval, Count of Bucquoy and the German Catholic League under Johann Tserclaes, Count of Tilly at Bílá Hora , near Prague. The battle marked the end of the Bohemian period of the Thirty Years' War.In the early 17th Century most of the Bohemian estates, although under the dominion of the predominantly Roman Catholic Holy Roman Empire, had large Protestant populations, and had been granted rights and protections allowing them varying degrees of religious and political freedom. In 1617, as Emperor Matthias lay dying, his cousin Ferdinand - a fiercely devout Roman Catholic and proponent of the Counter-reformation - was named his successor as Holy Roman Emperor and King of Bohemia. This led to deep consternation among many Bohemian Protestants, who feared not only the loss of their religious freedom, but also of their traditional semi-autonomy, under which many of the estates had separate, individual constitutions governing their relationship with the Empire, and where the King was elected by the local leaders.

Ferdinand (who would become Emperor Ferdinand II following Matthias' death in 1619) saw Protestantism as inimical to the Empire, and wanted to impose absolutist rule on Bohemia while forcefully encouraging conversion to the Roman Catholic faith. He also hoped to reclaim church properties which had been seized by Protestants at the start of the Reformation decades earlier, and to do away with the Electorate - the body of princes who chose the Holy Roman Emperor and who had considerable power over Imperial policy.

In 1620, now fully established as Emperor, Ferdinand II set out to conquer Bohemia and make an example of the rebels. King Frederick and his military commander, Prince Christian of Anhalt, had organized a Protestant army of 30,000 men; Ferdinand countered with a force of 25,000, many of them seasoned soldiers, under the capable leadership of Field Marshal Tilly, a Roman Catholic Spanish-Flemish nobleman. Tilly's army enjoyed the advantage of including two of the most successful military leaders in European history - Tilly himself and the future General Wallenstein. Tilly's force was made up of two distinct groups: Imperial troops commanded by Charles Bonaventure de Longueval, Count of Bucquoy, and soldiers of the German Catholic League, directly under Tilly. All of the armies of the day employed numerous mercenaries, including, by some definitions, Tilly himself. Serving with the Catholic League as an official observer was the future "father of modern philosophy", René Descartes. After conquering most of western Bohemia, the Imperial army made for Prague, the Bohemian capital, then in rebel hands. The Bohemians attempted to block them by setting up defensive positions, which the Imperial army simply bypassed. Force-marching his men, Christian of Anhalt managed to get ahead of the Imperial army just before Prague. He thus gained an advantageous position on the "White Mountain", actually a low plateau, but had little time to set up defensive works. Enthusiasm for joining battle was low on both sides. After the reverses of the previous several weeks, Christian of Anhalt's army had been reduced to about 15,000 men, with little prospect of victory; the mercenaries on both sides had not been paid in months; and with winter approaching, cold, wet, weather made for less than ideal combat conditions.The Battle of White Mountain was more a skirmish than a full-fledged battle. The Bohemian army was no match for the Emperor Ferdinand's troops. The actual battle lasted only an hour and left the Bohemian army in tatters. Some 4,000 Protestants were killed or captured, while Imperial losses amounted to only about 700.

With the Bohemian army destroyed, Tilly entered Prague and the revolt collapsed. King Frederick with his wife Elizabeth fled the country (hence his nickname the Winter King), and many Bohemians welcomed the restoration of Roman Catholic rule. Forty-seven noble leaders of the insurrection were put on trial, and twenty-seven of them were executed in Prague's Old Town Square on what came to be called the "Day of Blood". Amongst those executed were Kryštof Harant and Jan Jesenius. Today, 27 crosses have been laid into the cobblestones as a tribute to those victims. An estimated five-sixths of the Bohemian nobility went into exile soon after the Battle of White Mountain, and their properties were confiscated. Before the war about 151,000 farmsteads existed in the Lands of Bohemian Crown, while by the year 1648 only 50,000 remained. At the same time the number of inhabitants decreased from three million to only 800,000. The Thirty Years War had only begun, and Bohemia was often the scene of much bloodshed. In 1621, the Emperor ordered all Calvinists and other non-Lutherans to leave the realm in three days or to convert to Roman Catholicism. The next year, he also ordered all Lutherans (most of whom had not been involved in the revolt) to convert or else leave the country. By 1627, Archbishop Harrach of Prague and Jaroslav Borzita of Martinice set out to convert the heretics, as they were termed, by peaceful means; most Bohemians converted, but a significant Protestant minority remained. Spanish troops, seeking to encircle their rebellious Dutch provinces, seized the Palatinate lands. With the prospect of Protestantism being overrun in Germany, Denmark entered the struggle. Sweden was to join the Protestant forces in 1630.