Late Medieval Poland
Queen Jadwiga was only ten years old in 1384 when her mother Elizabeth sent her north from Hungary to become the Queen of Poland. Elizabeth stayed behind to rule Hungary as regent for Jadwiga's older sister. By 1385, when she was twelve, Queen Jadwiga married Jogaila, the Duke of Lithuania, uniting Poland and Lithuania to form a bigger empire that could defend itself against the Holy Roman Empire to its west and the Russian empire to its east.
The treaty that Queen Jadwiga and Duke Jogaila signed in 1385 AD
From 1385 AD on, Poland and Lithuania were united in one empire. Duke Jogaila converted to Christianity and took a new Polish name, becoming King Wladyslaw II. Wladyslaw and Jadwiga's empire covered not only modern Poland and Lithuania, but also a lot of what is now Belarus and Ukraine. This empire was big enough to defend itself successfully against Mongol invasions from the east and German invasions from the west. Jadwiga died of infection after childbirth in 1399, but Wladyslaw II ruled for an extremely long time - 48 years! - which gave him time to really lock Poland and Lithuania together as one empire.
King Wladyslaw II
By 1500 AD, the Polish-Lithuanian empire also controlled Bohemia and Hungary, which gave its kings control over pretty much all of eastern and central Europe. But in 1526, the Ottoman Empire attacked Hungary, and won a big battle there. The king of Hungary and Bohemia, Louis II, was killed in the battle, and as a result the Ottomans took over the eastern half of Hungary, while the Austrians were able to take over the western half. But Poland-Lithuania continued to be strong into the 1500s.