@Demetrios and DarthMaur
Mieszko II, son of Boleslaw I Chrobry (The Great) played important role in the fight about title of emperor between Konrad II and Konrad The Younger.
His wife was Rycheza, niece of Otton III and princess of Lotharingen. Henry II was the last emperor of Saxony dynasty and after his death in 1024, Konrad II of Salic dynasty was elected. But his cousin Konrad The Younger has similar rights, so fight started. Mother of The Younger was Matilda, who was in very cordial relations to Rycheza. Not long after adventure of Konrad II to Rome in 1026 rebellion had taken place. In 1028 Mieszko went to Saxony and was recognized by many local knights as a governor of Konrad The Younger. Many didn't want to break the word give nto Konrad II and fought against mieszko but like "Hildersheim Annales" says, Mieszko destroyed them and left great parts of Saxony devastated. In 1029 Emperor made an expedition but was unable to took the Budziszyn. In january of 1030 Mieszko II, in alliance with some saxon knights (led by Zigfrid, son of count Hodon) and paganic Lutikes (other name for Vielietians, but the latter name has wider sense) invaded Saxony and laid waste up to Sala and Laba rivers (bishop of Brandenburg was killed by pagans). German sources provides that he took 9.000 of captives. Konrad II eventually understood that he had to eliminate polish threat to secure his title. Mieszko had two brothers who got nothing after death of Boleslaw I (there was the patrimonial law, only the oldest son takes the hereditary). But in fact Bezprym was the oldest. Boleslaw disinherited him when he removed his mother from the court. Bezprym was sent to monastery of St. Romuald near Ravenna but fled probably in 1029. The younger brother Otto was dissatisfied about his role too and easily was convinced by Bezprym to join the plot. They colluded against Mieszko and went to emperor. Mieszko managed to destroy the plot in 1030 but instead killing the brothers, he exiled hem from Poland. In autumn of 1031 joint attack from emperor and from Kiev dethroned Mieszko II eventually. Konrad was morethan happy cause Mieszko personally with best soldiers fouhgt on the west border and emperor was about to lose. But Kievians with Bezprym were close to Visla, so Mieszko II was forced to cede Luzyce to emperor in truce treaty and hasted to the central Poland. It didn't help him anymore. Many people remembered that Bezprym was older and took his side. Moreover Mieszko lost Rycheza, who was very important. They quarelled harshly before. In fact Mieszko took the advantage and sent her son Kazimierz to the monastery (what meant that he didn't recognize him as a legal heir) in 1026. In 1031 Rycheza betrayed him and helped Bezprym to set contacts with Germany. Mieszko fled to Bohemia and Bezprym was recognized legal Duke of Poland. That was the price of friendship of emperor – he had to resignate from the crown (sent it to emperor through Rycheza in 1032).
Crown was regained in fact in 1076 but only to 1079. Another King of Poland was coronated at last in 1295 (Przemysl II).
Btw, Wratislaw II was given a title of king of Bohemia (in 1086) only temporary to his death (1092). Kingdom of Bohemia was founded in 1198 (King Przemysl I Ottokar, ruler of Bohemia since 1192, king since 1198, died in 1230).