<P> The Bohemians, desperate for allies against the emperor, applied to be admitted into the Protestant Union, which was led by their original candidate for the Bohemian throne, the Calvinist Frederick V, Elector Palatine . The Bohemians hinted Frederick would become King of Bohemia if he allowed them to join the Union and come under its protection . However, similar offers were made by other members of the Bohemian Estates to the Duke of Savoy, the Elector of Saxony, and the Prince of Transylvania . The Austrians, who seemed to have intercepted every letter leaving Prague, made these duplicities public . This unraveled much of the support for the Bohemians, particularly in the court of Saxony . In spite of these issues surrounding their support, the rebellion initially favoured the Bohemians . They were joined in the revolt by much of Upper Austria, whose nobility was then chiefly Lutheran and Calvinist . Lower Austria revolted soon after, and in 1619, Count Thurn led an army to the walls of Vienna itself . Moreover, within the British Isles, Frederick V's cause became seen as that of Elizabeth Stuart, described by her supporters as "The Jewell of Europe", leading to a stream of tens of thousands of volunteers to her cause throughout the course of the Thirty Years' War . In the opening phase, an Anglo - Dutch regiment under Horace Vere headed to the Palatinate, a Scots - Dutch regiment under Colonel John Seton moved into Bohemia, and that was joined by a mixed "Regiment of Brittanes" (Scots and English) led by the Scottish Catholic Sir Andrew Gray . Seton's regiment was the last of the Protestant allies to leave the Bohemian theatre after tenaciously holding the town of Třeboň until 1622, and only departing once the rights of the citizens had been secured . </P> <P> In the east, the Protestant Hungarian Prince of Transylvania, Gabriel Bethlen, led a spirited campaign into Hungary with the support of the Ottoman Sultan, Osman II . Fearful of the Catholic policies of Ferdinand II, Gabriel Bethlen requested a protectorate by Osman II, so "the Ottoman Empire became the one and only ally of great - power status which the rebellious Bohemian states could muster after they had shaken off Habsburg rule and had elected Frederick V as a Protestant king". Ambassadors were exchanged, with Heinrich Bitter visiting Constantinople in January 1620, and Mehmed Aga visiting Prague in July 1620 . The Ottomans offered a force of 60,000 cavalry to Frederick and plans were made for an invasion of Poland with 400,000 troops, in exchange for the payment of an annual tribute to the sultan . These negotiations triggered the Polish--Ottoman War of 1620--21 . The Ottomans defeated the Poles, who were supporting the Habsburgs in the Thirty Years' War, at the Battle of Cecora in September--October 1620, but were not able to further intervene efficiently before the Bohemian defeat at the Battle of the White Mountain in November 1620 . Later, Poles defeated the Ottomans at the Battle of Chocim and the war ended with a status quo . </P> <P> The emperor, who had been preoccupied with the Uskok War, hurried to muster an army to stop the Bohemians and their allies from overwhelming his country . Count Bucquoy, the commander of the Imperial army, defeated the forces of the Protestant Union led by Count Mansfeld at the Battle of Sablat, on 10 June 1619 . This cut off Count Thurn's communications with Prague, and he was forced to abandon his siege of Vienna . The Battle of Sablat also cost the Protestants an important ally--Savoy, long an opponent of Habsburg expansion . Savoy had already sent considerable sums of money to the Protestants and even troops to garrison fortresses in the Rhineland . The capture of Mansfeld's field chancery revealed the Savoyards' involvement, and they were forced to bow out of the war . </P> <P> The Spanish sent an army from Brussels under Ambrogio Spinola to support the Emperor . In addition, the Spanish ambassador to Vienna, Don Íñigo Vélez de Oñate, persuaded Protestant Saxony to intervene against Bohemia in exchange for control over Lusatia . The Saxons invaded, and the Spanish army in the west prevented the Protestant Union's forces from assisting . Oñate conspired to transfer the electoral title from the Palatinate to the Duke of Bavaria in exchange for his support and that of the Catholic League . </P>

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