Henry invades Poland on behalf of Zbigniew, again with the support of Svatopluk, who leads a Bohemian army across the Sudetes into Silesia to join the German forces at the Battle of Głogów.

The German forces had gathered in late summer 1109 at Erfurt, crossed the Polish border near Krosno on the Bóbr river and on St. Bartholomew's Day approached the fortified town of Glogów with the support of Svatopluk, whose troops arrive in September).

Defeating a Polish army stationed near the town, and knowing that Boleslaw III is in the city, Henry decides to grant Glogów's citizens a five day ceasefire to ask their king to surrender.

Henry makes the citizens of Glogów give up their sons as hostages as a guarantee of the ceasefire and promises to give them back alive, no matter what the answer of the Polish king will be.

Receiving no reply after the time limit passes, Henry lays siege to the city and, breaking his promise, chains the hostages to his siege engines, gambling that the people of Glogów will not shoot their own children.

His cruelty only strengthens the resolve of Glogów's defenders, who repulse several attacks by the Imperial army.

During the siege, Svatopluk is assassinated on September 21, 1109 in the tent of the emperor by a member of the family of Vršovice, whose chief, Mutina, he had decapitated for the support he had given to Borivoj, who is now returned to the Bohemian ducal throne.

After many days of unsuccessful fighting, Henry is forced to abandon the siege and march south.