In 1385, the Grand Duke Jogaila accepted Poland’s offer to become its king. Jogaila embarked on gradual Christianization of Lithuania and established a personal union between Poland and Lithuania. Lithuania was one of the last pagan areas of Europe to adopt Christianity.
After two civil wars, Vytautas the Great became the Grand Duke of Lithuania in 1392. During his reign, Lithuania reached the peak of its territorial expansion, centralization of the state began, and the Lithuanian nobility became increasingly prominent in state politics. In the great Battle of the Vorskla River in 1399, the combined forces of Tokhtamysh and Vytautas were defeated by the Mongols. Thanks to close cooperation, the armies of Lithuania and Poland achieved a victory over the Teutonic Knights in 1410 at the Battle of Grunwald, one of the largest battles of medieval Europe.
In January 1429, at the Congress of Lutsk Vytautas received the title of King of Lithuania with the backing of Sigismund, Holy Roman Emperor, but the envoys who were transporting the crown were stopped by Polish magnates in autumn of 1430. Another crown was sent, but Vytautas died in the Trakai Island Castle several days before it reached Lithuania. He was buried in the Cathedral of Vilnius.
After the deaths of Jogaila and Vytautas, the Lithuanian nobility attempted to break the union between Poland and Lithuania, independently selecting Grand Dukes from the Jagiellon dynasty. But, at the end of the 15th century, Lithuania was forced to seek a closer alliance with Poland when the growing power of the Grand Duchy of Moscow threatened Lithuania’s Russian principalities and sparked the Muscovite–Lithuanian Wars and the Livonian War.
On 8 September 1514, Battle of Orsha between Lithuanians, commanded by the Grand Hetman Konstanty Ostrogski, and Muscovites was fought. According to Rerum Moscoviticarum Commentarii by Sigismund von Herberstein, the primary source for information on the battle, the much smaller army of Poland–Lithuania (under 30,000 men) defeated a force of 80,000 Muscovite soldiers, capturing their camp and commander. The battle destroyed a military alliance against Lithuania and Poland. Thousands of Muscovites were captured as prisoners and used as laborers in the Lithuanian manors, while Konstanty Ostrogski delivered the captured Muscovite flags to the Cathedral of Vilnius.
The Livonian War was ceased for ten years with a Truce of Yam-Zapolsky signed on 15 January 1582 according to which the already Polish–Lithuanian Commonwealth recovered Livonia, Polotsk and Velizh, but transferred Velikiye Luki to the Tsardom of Russia. The truce was extended for twenty years in 1600, when a diplomatic mission to Moscow led by Lew Sapieha concluded negotiations with Tsar Boris Godunov. The truce was broken when the Poles invaded Muscovy in 1605.
The Polish–Lithuanian Commonwealth was created in 1569 by the Union of Lublin. As a member of the Commonwealth, Lithuania retained its institutions, including a separate army, currency, and statutory laws – the Statute of Lithuania. Eventually Polonization affected all aspects of Lithuanian life: politics, language, culture, and national identity. From the mid-16th to the mid-17th centuries, culture, arts, and education flourished, fueled by the Renaissance and the Protestant Reformation. From 1573, the Kings of Poland and Grand Dukes of Lithuania were elected by the nobility, who were granted ever increasing Golden Liberties. These liberties, especially the liberum veto, led to anarchy and the eventual dissolution of the state.