Mieszko II Lambert (990 - 1034), also spelled Miezko II, was the duke and short-term king of Poland. He was the son of Boleslaw I the Brave and Enmilda, daughter of Dobromir, Duke of Lusatia. Their children were Kazimierz I the Restorer, Rixa of Poland, and Gertrude of Poland.

Mieszko II was very well educated for the period. He was able to read and write, and knew both Greek and Latin. He is unjustly known as Mieszko Gnusny (the "Lazy," "Stagnant" or "Slothful"). He received that epithet due to the unfortunate way his reign ended; but at the beginning he acted as a skillful and talented ruler. Before he became king in 1025, he probably served as his father's governor in Kraków, most likely from 1013, and reputedly built many churches.

Beginning 1028, he successfully waged war against Germany: he was able to repel the German army, and later even invaded Saxony. He allied Poland with Hungary, resulting in a temporary Hungarian occupation of Vienna. This war was probably prompted by family connections of Mieszko's in Germany who opposed Emperor Conrad II.

An understanding of what happened later requires an understanding of Mieszko's family. His older brother Bezprym was the son of an unknown Hungarian wife of Boleslaw's and was later expelled by Mieszko. He also had a younger brother, Otton. By Slavic custom, a father should divide his legacy among all his sons. However, since a kingdom cannot be divided, Mieszko's brothers received nothing from their father's legacy.

As Bezprym was the oldest son, many probably felt that he should have succeeded his father as king. Bezprym had, however, always been disliked by his father, as indicated by his name (the Piasts tended to give names such as Boleslaw, Mieszko and later Kazimierz, Wladyslaw and emperors' names such as Otto, Conrad and Heinrich: Bezprym was a commoner's name, which implied that Boleslaw did not wish Bezprym to succeed him). He was packed off to a monastery.
Mieszko's two brothers escaped abroad: Otton to Germany, Bezprym to Kievan Rus. Soon after, the German emperor and the grand duke of Kiev, Yaroslav I the Wise, made alliance and simultaneously invaded Poland.

Facing two enemies, Germany on the west and Rus on the east, Mieszko escaped to Bohemia, where he seems to have been castrated. Bezprym began his reign by sending his crown and regalia to Germany. Mieszko soon returned, but was forced to pledge fealty to the German Emperor, and Poland was divided among him, his brothers Otton and Bezprym, and a certain Thiedric (probably a nephew or cousin). Otton was killed by one of his own men, and Mieszko was able to reunite Poland.

What happened next is a mystery. Historians now think that Mieszko was killed (1034) in a plot hatched by the aristocracy.

After Mieszko's death, Poland's peasants revolted in a "pagan reaction." The exact reasons and date are unknown. Mieszko's son, Kazimierz I the Restorer, was either expelled by this insurrection, or the insurrection was caused by the aristocracy's expulsion of him.

Modern historians argue that the insurrection was less likely caused by religious than by economic matters (huge new taxes for the Church, the militarization of the early Polish dukedom/kingdom — almost all the male population were drafted into the army — etc.). Priests, monks and knights were killed; cities, churches and monasteries were burned.

The chaos became still greater when unexpectedly the Czechs invaded from the south. The land became divided among local rulers, one of whom is known by name: Maslaw ruler of Masovia). Greater Poland was so devastated that it ceased to be the core of the Polish kingdom. The capital was moved to Kraków in Lesser Poland.