Jan Žižka
The first anti-Hussite crusade

King Sigismund was king of Hungary but only the titular king of Bohemia. Sigismund had acquired a claim on the Bohemian crown, though it was then in question (and remained so till much later) whether Bohemia was a hereditary or an elective monarchy, especially as the line through which Sigismund claimed the throne had accepted that the Kingdom of Bohemia was an elective monarchy elected by the nobles, and thus the regent of the kingdom (Čeněk of Wartenberg) also explicitly stated that Sigismund had not been elected as reason for Sigismund's claim to not be accepted. A firm adherent of the Church of Rome, Sigismund was successful in obtaining aid from Pope Martin V, who issued a bull on 17 March 1420 which proclaimed a crusade "for the destruction of the John Wycliffe, Hussites and all other heretics in Bohemia". Sigismund and many German princes arrived before the walls of Prague on 30 June at the head of a vast army of crusaders from all parts of Europe, largely consisting of adventurers attracted by the possibility of pillage. They immediately began a siege of the city and Žižka was compelled to defend the Kingdom. He was a pragmatist in developing his military strategy. His army consisted of farmers and peasants, lacking both the funds and equipment to be classic soldiers with sword, horse and armor, so Žižka used their farmers' skills to boost their military efficiency. He adapted tools of agriculture into tools of war. The agricultural flail was transformed into the flail.

Menaced by Sigismund, the citizens of Prague entreated the Taborites for assistance. Led by Žižka and their other captains, the Taborites set out to take part in the defence of the capital. At Prague, Žižka and his men took up a strong position on the hill just outside the city known as the Vítkov, now in Žižkov, a district of Prague named after the battle in his honour. On 14 July the armies of Sigismund made a general attack. A strong German Crusader-led force assaulted the position on the Vítkov, the stronghold that secured the Hussite communications with the open country. Thanks to Žižka's personal leadership, the attack was thrown back and the forces of Sigismund abandoned the siege. On August 22 the Taborites left Prague and returned to Tábor. Though Sigismund had retired from Prague, the castles of Vyšehrad and Hradčany remained in possession of his troops. The citizens of Prague laid siege to the Vyšehrad (see Battle of Vyšehrad), and towards the end of October the garrison was on the point of capitulating through famine. Sigismund attempted to relieve the fortress, but was decisively defeated by the Hussites on November 1 near the village of Pankrác. The castles of Vyšehrad and Hradčany now capitulated, and shortly afterwards almost all Bohemia fell into the hands of the Hussites.

Žižka now engaged in constant warfare with the partisans of Sigismund, particularly with the powerful Romanist, Oldřich II of Rožmberk. Through this struggle, the Hussites obtained possession of the greater part of Bohemia from Sigismund. It was proposed to elect the Grand Duke of Lithuania Vytautas to the throne. However, the estates of Bohemia and Moravia met at Čáslav on 1 June 1421 and decided to appoint a provisional government, consisting of twenty members chosen from all the political and religious parties of the country. Žižka, who took part in the deliberations at Čáslav, was elected as one of the two representatives of Tábor.

Žižka summarily suppressed some disturbances on the part of a fanatical sect called the Adamites. He then continued his campaigns against the Romanists and the adherents of Sigismund, and having captured and rebuilt a small castle near Litoměřice (Leitmeritz), he retained possession of it, the only reward for his great services that he ever received or claimed. According to the Hussite custom he gave the Biblical name of Chalice (Kalich in Czech) to this new possession, and henceforth adopted the signature of Žižka of the Chalice. Jan Žižka did not capture any more properties for himself during the Hussite Wars. This fact was unusual for the time and distinguished Žižka from his contemporaries.

Later that year he was severely wounded while besieging the Rabí Castle, and lost the use of his remaining eye. Though now totally blind, he continued to command the armies of Tábor.
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