Nicaea, a city of great strategic importance located in Asia Minor, was the capital of the Byzantine Empire and the seat of the Patriarchate of Constantinople. Following the call to arms by Pope Urban II, many European nobles and commoners embarked on the First Crusade with the primary aim of recapturing Jerusalem from Islamic rule. Upon reaching Asia Minor, the Crusaders encountered the well-fortified city of Nicaea, which was defended by the Seljuk Turks. The siege began on May 14, 1097, following several skirmishes outside the city walls. The Crusaders, numbering around 10,000 to 15,000, were led by prominent leaders such as Godfrey of Bouillon and Raymond IV of Toulouse, who recognized the vital need to secure Nicaea as it served as a gateway to the heart of Asia Minor and beyond.

During the months of the siege, the Crusaders deployed various tactics, including a combination of building siege towers and employing siege engines to breach the fortified walls. The defenders, led by the Seljuk Turks, put up fierce resistance, resulting in a protracted and arduous siege that tested the will and resources of the Crusader forces. Meanwhile, the Byzantine Empire's interest in the siege was significant as a successful capture of Nicaea would restore imperial influence over the region. Furthermore, it allowed the Byzantine Emperor Alexios I Komnenos to regain a stronghold, ensuring that the routes to the Holy Land would be safer for the Crusaders.

The siege culminated in June 1097 when negotiations between the Byzantine forces and the Crusaders led to a strategic collaboration that resulted in the capture of Nicaea. The city fell on June 19, 1097, marking one of the significant early victories of the First Crusade. The aftermath of the siege saw the Crusaders well fortified and better equipped for their journey towards Jerusalem, an endeavor that would come with its own challenges and triumphs in the months that followed.