The new kingdom created by the alliance of Lithuania and Poland immediately became the most powerful state in Eastern Europe. The arrangement guaranteed that Lithuania would still maintain its own separate identity with a Grand Prince of its own that may or may not at the time also be the King of Poland. Such was the strength of the state that it secured a great victory against the Teutonic Knights at Grunwald in 1410.
In 1501, an agreement, the Union of Lublin, was reached stating that the King of Poland would always also be the Grand Prince of Lithuania, a situation which continued until 1569 when there occurred a more complete merging of the two regions with the creation of a joint parliament made up of nobles from both areas.
During the renaissance which took place in Europe during the 16th century, there were innovations in the arts and styles of architecture as well as a renewed interest in the classical civilisations of Rome and Greece. The legacy of this period of development can be seen today in many of the beautiful buildings which survive in Lithuania from this time, particularly in Vilnius and Kaunas. Following this union, the influence of Poland on Lithuania began to increase.
The Lithuanian upper classes and the church began to use the Polish language and this usage was to filter down into public life, so much so that in 1696 Polish was to become an official language in replacement of the Lithuanian language. Despite the close union between Poland and Lithuania, however, Lithuania still retained certain autonomy and was still known as the Grand Duchy of Lithuania as part of the Polish-Lithuanian Commonwealth. Despite the shared parliament (sejm), Lithuania had separate laws and its own army and treasury.
In May 1791, government attempts were made to integrate the two countries more closely but separation was maintained by the Mutual Declaration of the Two Nations. However, partitions to the Polish-Lithuanian Commonwealth in the 1790s meant that Lithuania was divided up between Russia and Prussia and it ceased to exist as a distinct country for over a century.