The siege of Pilsen started on September 19, 1618, and ended on November 21, 1618. Now called Plzeň and located in the Czech Republic, Pilsen was then a city in the Kingdom of Bohemia, a land which had just been incorporated into the growing Habsburg Monarchy and its ruler, Emperor Charles V of both the Habsburg Dynasty and the Holy Roman Empire. This incorporation was significant due to the fact that the Habsburg Dynasty (and thus also the Holy Roman Empire) was officially Catholic while the Bohemian nobility was largely Protestant and, though Bohemia was already a part of the Holy Roman Empire itself, being ruled directly by the Catholic Habsburg Monarchy was seen as going too far. During a rebellion of Protestant Bohemian nobles called the Defenestration of Prague on May 23, 1618, nobles threw their imperial governors from the windows of Prague Castle and Catholic refugees fleeing the city took shelter in the nearby city of Pilsen. The Protestant forces decided to take Pilsen before the Imperial forces would be able to reinforce it.
The city was defended by the Count of Bucquoy, a French-born soldier who had distinguished himself while fighting for the Spanish King in the Netherlands. The city was well supplied to withstand a lengthy siege and had 4,000 burghers and 158 horsemen to defend it. The Protestant army assembled to take Pilsen was considerably larger. With 20,000 men under the command of Count Ernst von Mansfeld, an able military leader who had gained his experience fighting the Ottoman Empire in Hungary and in the War of the Jülich Succession, the Protestants were confident that they could starve Pilsen into submission before the Imperial army arrived to save it.
Both sides’ infantry carried a mixture of muskets and pikes, with the pikemen still using armor to protect themselves. Cavalry relied on firearms and used armor according to the role in which they served. Both sides had artillery, but while the Catholics lacked enough gunpowder and ammunition, the Protestants cannons were not heavy enough to breach the city's walls. Covered by the Mies River on the north and surrounded by flat ground to the south, the city had good defenses but was seriously undermanned. When Mansfeld arrived he settled down for a siege, deciding that he lacked the strength to take it by storm.
When Mansfield reached the outskirts of Pilsen, the defenders blocked the three gates which gave access to it. Though the Protestant forces had decided on a siege since they did not have the numbers or artillery to assault the city directly, on October 2nd Mansfeld's artillery arrived at the camp. The Protestant artillery battered the city's walls but failed to make an impression due to the small caliber of the guns. Finally, on November 21st, several parts of the walls were breached, and Mansfeld's soldiers took the city after several hours of vicious melee combat. The Protestants had only suffered 1,100 casualties in terms of the dead, wounded, or missing, while the Catholics suffered 2,500.
Pilsen was the first battle of a war that would last decades and cost the lives of millions of people. By attacking the city, the Protestant nobles closed the window for negotiations and precipitated a military response by the Emperor, helping to radicalize a local revolt into an all-out war. The most immediate consequence of the siege was the creation of an alliance of Catholic princes with the Emperor with the purpose of crushing the rebellion. The armies of the Catholic League (which had actually been established previously, in 1609) defeated the Bohemian rebels and reestablished the authority of the Holy Roman Emperor. As the war wore on, it would evolve into one of the deadliest wars Europe had ever seen, and a greedy series of political decisions superseding the conflict between religious beliefs that had started it.