Because of the initial uncertainty regarding the fate of Wladyslaw, and also because of disputes between Polish and Lithuanian nobles, the practical accession of Casimir IV to the Polish throne was to take two years. The new king proved himself a consummate politician. He managed to secure the throne whilst acknowledging the equality of his two realms in a ‘fraternal union’ under his control. In Poland he faced the opposition of magnates headed by Bishop Zbigniew, but he was helped by the ‘young barons of the kingdom’ who had been summoned from Great Poland to lend support to the government of the king and his council. In the early years of his reign he also sought support from the cities. The king and the royal party successfully opposed the financial demands of the papal curia and countered clerical opposition to the royal nomination of bishops. The death of Cardinal Zbigniew in 1455 led to the defeat of the opposition from Little Poland. At the same time Casimir somewhat belatedly confirmed the privileges granted to the nobility by his predecessors, and then established his position in the kingdom with regard to the constitutional nature of the crown. Moreover, by granting separate privileges for several territories in Nieszawa at the beginning of the decisive war with the Teutonic Order, Casimir ensured that no new taxes could be raised or military levies called without the agreement of conventions of nobles known as land diets. He thus opened the way for the creation of parliamentary forms of consultation between the king and the privileged classes.
In Lithuania Casimir reigned with wide authority but not despotic power. He won the support of the Lithuanian lords with a charter granted in Vilnius in 1447 which endowed the boyars of the grand duchy with the same rights as those enjoyed by the Polish nobles, including individual freedoms, the right to hold their own courts and to trial by their peers, and the exemption of subject boyars from tribute for grand-ducal matters. He confirmed Lithuania’s borders as they had been during Vytautas’s reign and promised to give government offices only to Lithuanians. The border dispute between Lithuania and Poland was settled with a compromise: the grand duchy retained Volyn', and the kingdom kept Podolia. The war with Moscow was halted for a time by the peace of 1449. This was a turning-point which ended Lithuanian expansion into Rus'; a brief period of equilibrium ensued, after which Muscovite expansion began in earnest.
In Prussia and Pomerania the Teutonic Ordensstaat was undergoing a period of internal crisis. Wealthy cities such as Danzig, Torun (Thorn) and Elblag (Elbing), with their German-speaking populations, opposed fiscal exploitation at the hands of the Order. Similarly the Order’s vassals of knightly rank, whether they spoke German or Polish, combined to found the Lizard League (Eidechsengesellschaft)). After 1440 the Prussian estates, the knights and the townsfolk joined together in the Prussian Union which, as representative of political society, conducted negotiations, mainly in matters of taxation, with the Order’s grand master. The repression of the Union at the hands of the Order provoked a revolt. In 1454 Casimir IV received a rebel delegation led by Hans von Baisen and, invoking the rights of the crown, he promulgated the incorporation of Prussia into the kingdom. A Thirteen Years War then erupted, to be waged without Lithuanian assistance. International opinion did not favour the abolition of the Teutonic Order, and Popes Calixtus III and, after him, Pius II inter-vened by placing Poland under an interdict which the people and clergypromptly ignored. The war endedin 1466 with the Peace of Torun. Danzig and Pomerania, which had been lost to the Order in 1308, and the western part of Prussia, including Elblag and Malbork (Marienburg), were ceded to the kingdom of Poland to be known henceforth as Royal Prussia. The Order’s power was restricted to the remainder of Prussia, excluding Warmia (Ermland) which was held by the local bishop as a vassal of the Polish king. The grand master of the Order transferred his capital from the castle in Malbork to Konigsberg and swore allegiance to the Polish king whom he acknowledged as the Order’s sovereign.
In addition to this considerable restitution of crown lands, a little earlier Poland had made some territorial acquisitions on the Silesian border which were important because of their proximity to Cracow. In 1457 the king gained the duchy of Oswiecim and sovereignty over the duchy of Zator which would become a crown possession in 1497. The crown also received a part of the vassal duchy of Mazovia following the extinction of certain branches of the Mazovian Piasts (in 1462 and 1476).