From the beginning, his rule was contested by Otto II the Black, the younger brother of Svatopluk, who gained support not only from Vladislaus's widow Richeza of Berg, but also from King Lothair III of Germany.

Soběslav therefore was an adversary of the German monarchy throughout his reign and began by using the divisiveness of the 1125 election of King Lothair against rival Duke Frederick II of Swabia to further Bohemian independence.

Lothair III declared that no one could succeed to the Bohemian throne without Imperial investiture and proceeded to invade on behalf of Otto II.

On 18 February 1126, the German and Moravian troops under Lothair met the Bohemian forces at the Battle of Chlumec, a frontier fortress at the border with the March of Meissen.

At the Reichstag of Bamberg in the same year, he gave Soběslav the hereditary dignity of the archcupbearer of the Holy Roman Empire and the promise to vest his eldest son Vladislaus with Bohemia upon his death.