Boleslaus I (Czech: Boleslav I. Ukrutný; 915–972), a member of the Přemyslid dynasty, was ruler (kníže, "prince") of the Duchy of Bohemia from 935 until his death in 972.

Boleslaus I is generally respected by Czech historians as an energetic ruler who significantly strengthened the Bohemian state and expanded its territory.

Certain medieval historians, notably Cosmas of Prague, overtly present Boleslaus as a cruel figure, particularly after the death of his brother.

[3] Vratislaus took over the rule in Prague around the time of Boleslaus' birth, during which he had to manage both the exertion of influence by the East Frankish dukes of Bavaria and Saxony and the Magyar incursions.

In 935 (or in 929, according to other sources), Wenceslaus was murdered at Stará Boleslav after accepting an invitation from Boleslaus to celebrate the feast of Saints Cosmas and Damian with him there.

[4] Historians are uncertain of Boleslaus I’s motives for the assassination, but argue it was likely a desire for power, the influence of his pagan mother, or to put an end to Wenceslaus I’s political plans.

[2][4][3] Regardless of his specific motive, it was a planned political move that put Boleslaus I in power as Duke of Bohemia and opened the door for immense changes in the ruling style of the Přemyslid Dynasty.

Some historians claim that Boleslaus played a large role in promoting the cult because the sacredness of the canonization would be an advantage to his rule.

This war rejuvenated a newfound sense of unity within the kingdom as they came together to fight against the East Frankish Empire pushing them out of Prague.

Five years after the war with Otto, Boleslaus and the armies of the Bohemians and Franks allied against the Magyars in the victorious Battle of Lechfeld.

He had also helped Otto to crush an uprising of Slavic tribes led by the Obotrite princes Nakon and Stojgněv on the Lower Elbe river in the Battle on the Raxa.

[5] As a result of the victory, Boleslaus freed the Moravian lands from Magyar raids and expanded his territory, which in turn was later conquered by Polish dukes and became known as Upper Silesia and Lesser Poland.

[8] Flour, tin, slaves, and furs were desired among the Rus, Slavs, Turks, and Jews who are recorded to have traveled to Bohemia to trade for these goods.

The expansive system of trade and production in Prague indicates cultural connections and foreign relationships that existed during the reign of Boleslaus I.

[4] It is unknown if she was the mother of all four of his adult children: To defuse the Bohemian-Polish conflict, Duke Boleslaus married his daughter Doubravka to the pagan Piast prince Mieszko I in 963/964.

Boleslaus I’s daughter, Mlada, was raised as a devout Cristian, allegedly having close ties with the pope, and was taught monastic discipline.