Casimir III the Great (Polish: Kazimierz III Wielki; 30 April 1310 – 5 November 1370) reigned as the King of Poland from 1333 to 1370. He also later became King of Ruthenia in 1340, retaining the title throughout the Galicia–Volhynia Wars. He was the last Polish king from the Piast dynasty.
Casimir's reign was marked by a pragmatic foreign policy that prioritised stability over military expansion. He strengthened the Polish army and normalised relations with the Kingdom of Bohemia as well as the Teutonic Order, often through treaties and legal arbitration rather than war. While accepting the loss of Silesia to Bohemia stemming from ducal fragmentation, he successfully expanded Polish influence eastward, annexing Red Ruthenia, which increased the kingdom's territory, population, and economic potential. These diplomatic choices allowed Poland to avoid costly conflicts and focus on internal consolidation.
Domestically, Casimir undertook extensive legal and administrative reforms that laid the foundations of a more centralized state. He codified Polish law in the Statutes of Wiślica and Piotrków, reducing legal division and reinforcing royal authority. His reforms of the judiciary gained him the unofficial title "the Polish Justinian". His reign saw the growth of towns under Magdeburg rights, improvements in taxation and administration, and significant investment in infrastructure, including the construction of stone castles and fortified towns. Casimir was also known for confirming and extending protections for Jewish communities, building on earlier privileges and fostering economic development through trade and finance. He also encouraged the Jews to settle in great numbers.
Casimir III was a notable patron of learning and culture, most famously founding the University of Kraków in 1364, which became a lasting center of scholarship in Central Europe and remains one of the oldest institutions of higher learning in the world. Despite his achievements, he left no legitimate male heir, and upon his death the Polish crown passed to his nephew, Louis I of Hungary, thus ending the centuries-long Piast dynasty's rule and introducing a personal union with Hungary. Casimir's legacy endured, however, in the strengthened institutions, expanded territory, and relative prosperity of the Polish state, securing his reputation as one of Poland's most effective and influential medieval monarchs.
Casimir was born on 30 April 1310 in Kowal, Kuyavia, the third son of Władysław I Łokietek (Ladislaus the Short) and Jadwiga of Kalisz. At the time of Kazimierz's birth, his father was one of the most powerful Polish princes, ruling over most of Lesser Poland and Kuyavia, including the capital of the kingdom, Kraków. Ladislaus had royal ambitions, which were rekindled by the deaths in 1305 of Wenceslaus II and, a year later, Wenceslaus III, kings of Bohemia, who also ruled over a significant part of Poland and were kings of Poland. In 1311, the legal title to the Polish crown was taken over by the new Bohemia king, John of Bohemia, who, although he was never crowned, used the title of Polish king until 1335.
Through considerable military and diplomatic effort, Ladislaus managed to take control of a large part of the Polish lands and was crowned king in Kraków on 20 January 1320. A sign of his growing political importance was the marriage of his daughter Elizabeth to the Hungarian king Charles I of Hungary on 6 June 1320.
It was probably around that time that the king began to look for a bride for his son and only heir. According to some accounts, the first candidate was Jutta, daughter of John of Bohemia, but these plans never came to fruition. Subsequently, seeking allies against the Teutonic Order, the king turned to Gediminas, Grand Duke of Lithuania, who in 1323 announced his willingness to accept baptism. The marriage of Casimir to the Lithuanian princess Aldona of Lithuania took place on 16 October 1325, at Wawel Cathedral.
An important episode in Casimir's youth was his stay at the Buda court on a diplomatic mission around 1329/1330. There, he became involved in a romance with Klára Záh, the daughter of the Hungarian magnate Felician Záh. Klara's father learned of the affair only after Casimir had left Hungary. Seeking revenge for the dishonor brought upon his daughter, and accusing Queen Elizabeth, Casimir's sister, of complicity, he burst into Buda Castle during a royal feast and attacked the royal couple, wounding the king and cutting off the queen's fingers. Záh was killed on the spot, and his family suffered severe repressions.
Meanwhile, Poland became entangled in a war with the King of Bohemia, who sought to reclaim the Kraków throne, aided by Brandenburg and the Teutonic Order. Although Poland was supported by Lithuania and Hungary, the war did not go well for her. Władysław managed to defend his crown, yet the overall outcome of the conflict was unfavourable. The Teutonic Order seized Kuyavia, a move later confirmed by John of Bohemia, while John himself occupied the Dobrzyń Land, driving out Ladislaus' cousins, and received homage from the Duke of Płock and most of the Silesian princes as King of Poland.
In 1331, Casimir was granted the governorship of Greater Poland and Kuyavia, thus becoming both a witness to and a participant in the wars that were going unfavorably for the kingdom. After concluding a truce with the Teutonic Knights, King Władysław went to Greater Poland to deal with the Dukes of Głogów, who ruled over the southern part of that province and supported the Teutonic Order. In the fighting, Casimir distinguished himself by leading his Polish-Hungarian unit in a successful assault on besieged Kościan, during which he carried out a massacre of its defenders.
At the time of his father's death on 2 March 1333, he inherited a small and weakened realm consisting of two ravaged provinces, Lesser Poland and Greater Poland, between which lay the Łęczyca and Sieradz lands, granted in lifetime administration to the Kuyavian Piasts as compensation.
Succession to the throne by Casimir met with no major obstacles. The king was crowned at Wawel Cathedral on 25 April 1333, eight weeks after his father's death. The coronation was performed by Archbishop Janisław of Gniezno, assisted by Bishop Jan Grot of Kraków and Bishop Jan Doliwa of Poznań. Casimir's wife, Aldona of Lithuania, was crowned alongside him. The king's mother, Jadwiga of Kalisz, withdrew to the convent of the Poor Clares in Stary Sącz.
The political situation of Poland, however, was very difficult. The Bohemia king, John of Bohemia, was still widely regarded as the rightful king of Poland and remained formally at war with Casimir. As such, he was the overlord of the Silesian dukes (with the exception of the Duchy of Świdnica) and the Duke of Płock, whose lands surrounded Casimir's small realm. The truce with the Teutonic Order had been extended before the coronation, but it was set to expire in mid-1334. Casimir's only strong asset was his alliance with Hungary. The alliance with Lithuania had weakened and was also politically inconvenient, as the Lithuanians still adhered to paganism.
In order to improve the political situation, Casimir launched a diplomatic campaign. He first extended the truce with the Teutonic Order for another year, and then, in July 1333, signed a peace treaty with Louis V, Duke of Bavaria, Margrave of Brandenburg, the emperor's son.
New prospects opened for Casimir after the death of Henry of Bohemia, Duke of Carinthia and Tyrol, former King of Bohemia, whose inheritance became the subject of rivalry among the Habsburgs, Luxembourgs and Wittelsbachs. Casimir skillfully maneuvered between these powerful German dynasties. In May 1335, he began negotiations for a marriage between his daughter and the emperor's younger son, Louis II, Elector of Brandenburg. The Luxembourgs, alarmed by this rapprochement, sent Charles, Margrave of Moravia and son of King John, to Kraków with a proposal to settle all disputes amicably.