Crusaders begin the siege of Nicaea, a mostly Christian city guarded by several thousand Turkish troops. Byzantine Emperor Alexius I Comnenus has a strong interest in the capture of this heavily fortified city because it lies just 50 miles from Constantinople itself.
In fact, the prospect of western Christian armies liberating Nicaea and returning it to Byzantine control was a primary reason why Alexius supported the launch of the First Crusade to begin with. Alexius and his predecessors had fought hard to get Nicaea back, but without success.
Nicaea is at this time under the control of Kilij Arslan (Dawud Kılıj Arslan ibn Süleyman ibn Kut al-Mish), sultan of the Seljuk Turkish state of Rhüm (a reference to Rome). Unfortunately for him Arslan and the bulk of his military forces are at war with a neighboring Emir when the crusaders arrive; although he quickly makes peace with the Emir in order to get back and lift the siege, he will be unable to arrive in time.