Casimir the Jagiellonian (Kazimierz Jagiellonczyk), second son of Vladislaus II (Vladislaus Jagietto) and Sophia (Sonka), daughter of Andrzej, Duke of Holsza, was born on 30th November 1427 in Cracow. He spent his childhood at his mother's court. Following the assassination in 1440 of Sigismund, Grand Duke of Lithuania, Casimir, who was just 12 at the time, was sent out to Lithuania to act as viceroy for his brother, Vladislaus III, King of Poland and Hungary. He was immediately elected Grand Duke by the lords of Lithuania. This act was formally a breach of the act of union between Poland and Lithuania. However, thanks to a series of skilful compromises and an ability to benefit from the rivalry between the various aristocratic houses of Lithuania, in a short time Casimir managed to calm the storm somewhat and consolidate his own position. Five years later, when it was obvious that his elder brother King Vladislaus III had died at the Battle of Varna, an embassy from the lords of Poland, including the powerful Bishop Zbigniew Olesnicki, invited Casimir to ascend the throne of Poland. Casimir did not accept until September 1446, but on his own conditions. He was crowned King of Poland in Cracow on 25th June, 1447. After his coronation he renewed the personal act of union between the Kingdom of Poland and the Grand Duchy of Lithuania, in a deed which laid down principles of partnership between the two parts of his realm. In his concern for Lithuania he never neglected Poland and was a successful defender of the Polish raison d'etat. This was most manifest during his thirteen-year war with the Teutonic Order. The war broke out in 1454, when the Prussian Union, which had rebelled against the Teutonic Order, and made an act of homage to Casimir on behalf of all the lands of Prussia, including Gdansk, Toruii, and even Konigsberg (now Kaliningrad; Polish Krolewiec, 16th-century English name Regiomont). Following this transfer of feudal fealty, Casimir announced the incorporation of Prussia in the Kingdom of Poland. The war started with a defeat of Poland at Chojnice, where the general levy of the nobility and gentry called to arms and fighting under the King's command, sustained a severe setback owing to the more advanced combat techniques used by the enemy, which reduced their battle worthiness.

Casimir the Jagiellonian was always up against shortage of funds. He could never secure a constant source of revenue, and during his reign the crown estates were gradually shrinking. He was able to maintain an army only in wartime, when Sejm passed a tax bill to fund a mercenary force. It was during his reign that the parliamentary system enjoyed by the nobility and gentry was developed. Its basis were the royal privileges of Cerekwica and Nieszawa granted in 1454 to the nobility and gentry assembled for the military campaign. They provided that the monarch could not summon a general levy of military forces, nor issue new laws, without the consent of the sejmiki, or regional assemblies. From that time on the status of the sejmiki increased tremendously, as thanks to the privileges of Nieszawa these local assemblies were endowed with the power to influence crucial decisions of state. In view of the problems with getting the various regional sejmiki to agree on a single, joint position, the general sejm, which developed out of the national assemblies, grew in importance. The king was well aware of the need of a mercenary (professional) infantry and artillery, essential if he was to succeed in taking the heavily fortified Teutonic castles. This called for a substantial financial outlay. Casimir borrowed money from the lords and magnates, and in return endowed them with his royal estates by way of pledge on the loans. He took similar loans from the cities, granting them new privileges in return. In this way Casimir not only managed to get the money needed to finance his own troops, but also paid out the Teutonic Order's arrears to the soldiers garrisoning Malbork (Marienburg), the Order's principal stronghold. In return they opened up the gates to let him into the castle.

The effective operations and successes of the Polish army under the command of Piotr Dunin, and the scattering of the Teutonic fleet by ships sent by the cities of Gdansk and Elblqg turned the scales in Casimir's favour. In 1466 the war against the Order ended in victory for Poland and a peace treaty at Torun, on the strength of which the western part of the Teutonic state, henceforth known as "Royal Prussia" (as against "Monastic Prussia"), along with the dominions of the Bishop of Varmia (including the cities of Lidzbark and Olsztyn) became part of the Kingdom of Poland. The entire course of the Vistula now lay within the borders of the Kingdom of Poland, which gained access to the sea along with the significant ports of Gdansk and Elblqg. This meant a fundamental change to Poland's economic situation, but also to her military and political standing. The Thirteen Years' War against the Teutonic Order confirmed Casimir's authority, bringing him the reputation of a military victor and ruler of a consolidating state. He was able to securely hold on to his gains. When Mikotaj Tungen, Bishop of Varmia, who did not enjoy Casimir's support, enlisted the help of Matthias, King of Hungary and the Teutonic Knights to start a war (known as the Priests' War) against Poland, Casimir sent Polish troops into Varmia, forcing the Bishop into submission and pleading for mercy. The Grand Master of the Teutonic Knights, who had assisted the Bishop, was compelled to take the oath of fealty to Casimir, over which he had procrastinated hitherto. Casimir also used peaceful means to expand his realm. In 1456 he purchased the Duchy of Oswi^cim and took the Duchy of Zator under his tutelage (later both would be incorporated in Poland). In 1462 when a line of dukes of Mazovia became extinct, he incorporated the Duchy of Betz, and the Land of Rawa and Gostynin in the Kingdom of Poland, along with the Land of Sochaczew in 1476. However he could not manage to retrieve Silesia, since he did not want to engage in a war against the Czechs, with whom he had an alliance.

For the whole of his long reign Casimir the Jagiellonian was resolute in his endeavours to strengthen his position on the domestic scene. Continually on the brink of open conflict with the lords of Little Poland, led by Zbigniew Olesnicki, Bishop of Cracow, who were in opposition to him, and who at the beginning of his reign made up an important part of his privy council, he sought the support of the middle gentry and set up a new body of advisers. Making use of his royal prerogative to choose his servants, he appointed individuals who were loyal to him to the important offices of state, including his council. They helped him carry out his plans to centralise the state and accomplish the necessary treasury and military reforms. At the same time he engaged in a battle for the right to nominate bishops, and won. This was important, as bishops were members of the royal council and held high offices in the state. Casimir was good at sugaring his opponents' pill whenever he came out victorious in a conflict. He would distribute offices, estates, or revenues to those who had challenged him and lost, thereby counteracting their opposition and gaining new friends. In particular he wanted to have the support of the whole of the Church.

Casimir devoted a considerable amount of attention to dynastic matters. Thanks to his marriage with Elisabeth of Habsburg, later referred to as "the mother of kings", the ancient but provincial house of the Jagiellons achieved an illustrious status in Europe. The numerous daughters born of this union were given in marriage to the princes of the neighbouring states, giving rise to beneficial political alliances. Poland's strengthening as a power enabled Casimir to put his eldest son Wtadystaw on the throne of Bohemia in 1471, and on the throne of Hungary in 1490. Thereby in the late 15th century members of the Jagiellonian dynasty held the crowns of the three major kingdoms in East-Central Europe, which was a colossal boost to their prestige. However it was not Casimir's ambition to create a power consisting of four component parts, but rather to establish a bond based on a united family background.

One of Casimir's merits was his concern for the security of his realm. He appreciated the importance of a navy, and on the basis of an agreement with the ships of Gdansk established the germs of a Polish navy, whose potential was well proved during the Thirteen Years' War. He was not able to halt the expansion of the Grand Duchy of Moscow, which was growing in power, but at a cost of a small territorial loss on Lithuania's border with Russia he managed to mitigate border conflicts and maintain peace in the eastern part of his realm. He made a truce with Turkey and managed to maintain Polish suzerainty over Moldavia.

Casimir was prudent and circumspect, and did much to stabilise and modernise his realm, endeavouring to give it firm new foundations. He was one of the most outstanding kings of Poland. His reign made a permanent contribution to Polish history thanks to the recuperation of the Vistula estuary. He made Poland flourish economically and culturally. It was during Casimir's reign that Jan Dtugosz wrote his history of Poland, and Veit Stoss created his masterpiece, the carved high alter in St. Mary's, Cracow. In the University of Cracow humanism was gradually superseding scholasticism, launching the new trends in learning and the liberal arts. Casimir IV died on 7th June 1492 at Grodno. His Late Gothic tombstone and effigy, by Veit Stoss, is in Wawel Cathedral, Cracow.

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