The siege of Nicaea was the first major battle of the First Crusade, taking place from 14 May to 19 June 1097. The city was under the control of the Seljuk Turks who opted to surrender to the Byzantines in fear of the crusaders breaking into the city. The siege was followed by the Battle of Dorylaeum and the Siege of Antioch, all taking place in modern Turkey.

The First Crusade (1096–1099) was the first of a series of religious wars, or Crusades, initiated, supported and at times directed by the Latin Church in the middle ages. The objective was the recovery of the Holy Land from Islamic rule. While Jerusalem had been under Muslim rule for hundreds of years, by the 11th century the Seljuk takeover of the region threatened local Christian populations, pilgrimages from the West, and the Byzantine Empire itself. The earliest initiative for the First Crusade began in 1095 when Byzantine emperor Alexios I Komnenos requested military support from the Council of Piacenza in the empire's conflict with the Seljuk-led Turks. This was followed later in the year by the Council of Clermont, during which Pope Urban II supported the Byzantine request for military assistance and also urged faithful Christians to undertake an armed pilgrimage to Jerusalem.