Poland-Lithuania from Wikipedia:
With the death of Casimir the Great the period of hereditary monarchy in Poland ended, as Casimir did not have any male heirs to succeed him. The land owners and nobles did not want a strong monarchy. A constitutional monarchy was established between 1370 and 1493.
During the reign of King Louis I Poland formed a union with Hungary. This union was known as the Privilege of Koszyce. This union lasted for twelve years and ended in war. The failure of the union of Poland and Hungary paved the way for the union of Lithuania and Poland.
In 1385, the Union of Krewo was signed between Louis' daughter Jadwiga and Jogaila, the Grand Duke of Lithuania (later known as Władysław II Jagiełło), beginning the Polish-Lithuanian Union and strengthening both nations in their shared opposition to the Teutonic Knights and the growing threat of the Grand Duchy of Moscow.
Between 1386 and 1572 Poland and Lithuania were ruled by a succession of constitutional monarchs of the Jagiellon dynasty. The political influence of the Jagiellon kings was diminishing during this period, which was accompanied by the ever increasing role in central government and national affairs of landed nobility. The royal dynasty however had a stabilizing effect on Poland's politics. The Jagiellon Era is often regarded as a period of maximum political power, great prosperity, and in its later stage the Golden Age of Polish culture.
The first king of the new dynasty was the Grand Duke of Lithuania Jogaila, or Ladislaus II as the King of Poland. He was elected a King of Poland after becoming a Christian and marrying Jadwiga of Anjou, daughter of Louis I, who was Queen of Poland in her own right. Władysław Jagiełło was later the victor in the famous Grunwald Battle. Two of the Jagiellon rulers after him enjoyed long and successful reigns. Casimir Jagiellończyk fought the Teutonic Order, and Sigismund I the Old supported the arts and built the presently existing Wawel Renaissance castle. Sigismund II had no children, thus ending Poland's last hereditary succession.
The personal union with the Grand Duchy of Lithuania located north-east of Poland paved the way for closer relations of the two states. By the Union of Lublin in 1569 a unified Polish-Lithuanian Commonwealth (Rzeczpospolita) was created, stretching from the Baltic Sea and the Carpathian mountains to present-day Belarus and western and central Ukraine (which earlier had been Kievan Rus' principalities).
The Teutonic Knights, in control of Prussia since the 13th century, were defeated by a combined Polish-Lithuanian force in 1410 in the Battle of Grunwald, and in the later Thirteen Years War. In the Second Peace of Thorn (1466) they had to surrender the western half of their territory to the Polish crown (the areas known afterwards as Royal Prussia), and to accept Polish-Lithuanian suzerainty over the remainder (the later Ducal Prussia).
During this period Poland became the home to Europe's largest Jewish population, as royal edicts guaranteeing Jewish safety and religious freedom, issued during the 13th century, contrasted with bouts of persecution in Western Europe. This persecution intensified following the Black Death of 1348–1349, when some in the West blamed the outbreak of the plague on the Jews. Much of Poland was spared from this disease, and Jewish immigration brought their valuable contributions and abilities to the rising state. The greatest increase in Jewish population occurred in the 18th century, when the Jews constituted up to 7% of the population.