Sobeslav of Bohemia is first documented about 1107, when he and his elder brother Duke Bořivoj II of Bohemia were expelled by their Premyslid relative Svatopluk of Olomouc and fled to the court of their maternal cousin Duke Boleslaw III Wrymouth of Poland.
After Svatopluk was assassinated during the Battle of Głogów in 1109, Borivoj's attempts to regain the Bohemian throne failed, as in the following fratricidal war his younger brother Vladislaus I prevailed, backed by King Henry V of Germany.
Later, the brothers reconciled and Sobeslav was vested with rule at Brno and Znojmo in Moravia from 1115 until 1123, when the tensions between the brothers rose again and Sobeslav was once more expelled.
Nevertheless, as the last surviving son of Vratislaus II, Sobeslav had succeeded to the ducal throne after Duke Vladislaus' death in 1125.
From the beginning, his rule had been contested by Otto II of Olomouc, the younger brother of Svatopluk, who had gained the support not only from Vladislaus' widow Richeza of Berg but also from King Lothair III of Germany.
Junior princes, throughout the Premyslid era, will often rule all or part of Moravia from Olomouc, Brno or Znojmo, with varying degrees of autonomy from the ruler of Bohemia.
When Sobeslav decided to remove Otto II from Olomouc, the despoiled prince had looked to the German king for recourse.
Lothair III, declaring that no one could succeed to the Bohemian throne without Imperial investiture, proceeded to invade on behalf of Otto II.
This, however, is dangerous to the interests of the local nobility, and they rally around Soběslav.
The German and Moravian troops under Lothair meet on February 18, 1126, with the Bohemian forces at the Battle of Chlumec, a frontier fortress at the border with the March of Meissen (either near Chlumec or near Jílové).
Sobeslav routs and captures King Lothair, while Otto II is killed in battle.
However, the relationship between the two countries returns to the former vassal-suzerain relation, as King Lothair is released on condition of Sobeslav’s investiture with Bohemia.
