First Mongol invasion of Poland: Mongols overwhelm Polish armies in Kraków in the Battle of Chmielnik and plunder the city.

The Mongol invasion of Poland from late 1240 to 1241 culminated in the Battle of Legnica, where the Mongols defeated an alliance which included forces from fragmented Poland and their allies, led by Henry II the Pious, the Duke of Silesia and High Duke of Poland. The first invasion's intention was to secure the flank of the main Mongolian army attacking the Kingdom of Hungary. The Mongols neutralized any potential help to King Béla IV being provided by the Poles or any military orders.

The Mongol Empire was the largest contiguous empire in history. Originating in present-day Mongolia in East Asia, the empire at its height stretched from the Sea of Japan to Eastern Europe, extending northward into the Arctic; east and southward into the Indian subcontinent, mounting invasions of Southeast Asia, and conquering the Iranian plateau; and reaching westward as far as the Levant and the Carpathian Mountains.

The period of rule by the Piast dynasty between the 10th and 14th centuries is the first major stage of the history of the Polish state. The dynasty was founded by a series of dukes listed by the chronicler Gall Anonymous in the early 12th century: Siemowit, Lestek and Siemomysł. It was Mieszko I, the son of Siemomysł, who is considered the founder of the Polish state. Mieszko converted to Christianity of the Western Latin Church in an event known as the Baptism of Poland in 966. His acceptance of Christianity marked the formal beginning of Polish statehood, which was recognized by the Papacy and other Christian states of Europe. The Papal incipit titled Dagome iudex, first defined Poland's geographical boundaries and affirmed that Mieszko was under the protection of the Apostolic See. Mieszko also completed a unification of the Lechitic tribal lands that was fundamental to the existence of the new country of Poland. The ruling house then remained in power in the Polish lands until 1370.