Following the death of Bolesław I the Brave, his expansive policies led to a strain on the resources of the early Polish state, culminating in a collapse of the monarchy. Recovery was initiated by Casimir I the Restorer, who ruled from 1039 to 1058. His son, Bolesław II the Generous, however, faced significant challenges during his reign from 1058 to 1079, including a notorious conflict with Bishop Stanislaus of Szczepanów. The bishop's murder by Bolesław, following his excommunication over allegations of adultery, incited a revolt by the Polish nobles, resulting in Bolesław's deposition and exile.

The fragmentation of Poland was further exacerbated after 1138 when Bolesław III, in his Testament, divided his realm among his sons, leading to diminished monarchical control and frequent internal conflicts throughout the 12th and 13th centuries. During this era, notable figures like Casimir II the Just in 1180 sought to strengthen their rule by aligning more closely with the Church, while the chronicler Wincenty Kadłubek provided additional historical insights around 1220.

The internal divisions made Poland vulnerable to external threats, exemplified by the Teutonic Knights' invasion at the behest of Konrad I of Masovia in 1226, initially to combat Baltic Prussian pagans but resulting in prolonged conflicts over territory. The Mongol invasions beginning in 1240 further destabilized the region, with the significant defeat at the Battle of Legnica in 1241. Despite these challenges, the period was also marked by economic growth and urban development, with Wrocław becoming the first incorporated Polish municipality in 1242 and numerous cities being established under Magdeburg Law.

Efforts to reunify Poland gained traction in the late 13th century, with Duke Przemysł II's brief reign as king in 1295 marking a short-lived restoration of the monarchy. It was not until Władysław I the Elbow-high ascended in 1320 that more substantial progress was made towards reunification. His son, Casimir III the Great, ruling from 1333 to 1370, significantly strengthened and expanded the Kingdom of Poland, although losses such as Silesia persisted.

Casimir III also furthered the integration of diverse populations, confirming in 1334 the privileges of the Jewish community established by Bolesław the Pious in 1264, thus encouraging Jewish settlements. His reign also saw the beginning of the conquest of Red Ruthenia in 1340 and the establishment of what would become Jagiellonian University in 1364, underscoring a period of significant cultural and territorial expansion despite ongoing challenges.