The Second Crusade (1145–1149) was instigated by Pope Eugenius III in response to the fall of the County of Edessa to the forces of the Muslim leader Zengi. The county had been founded during the First Crusade. This crusade was the first to be led by kings, namely Conrad III of Germany and Louis VII of France. The armies of the two kings marched separately across Europe. After crossing into Byzantine territory in the Balkans, both armies made their way towards the Byzantine capital, Constantinople. The crusader armies intended to then take the overland route across Asia Minor to reach the Holy Land.
Arrival of the Second Crusade before Constantinople, portrayed in Jean Fouquet
's painting from around 1455–1460, Arrivée des croisés à Constantinople
Conrad had insulted Manuel by calling him "King of the Greeks" rather than his formal title of "Emperor of the Romans", and the imperial pretensions of the Germans made them far more suspect in Byzantine eyes than were the French. Following oaths taken by the German leaders that they intended no injury to the Byzantine Empire, Manuel made preparations for markets to be made available as the crusader army crossed imperial territory. A Byzantine force under the experienced general Prosouch (Borsuq), who was of Turkish origin, shadowed the Germans. A minor clash between the Byzantine force and the crusaders occurred near Adrianople, with the Byzantines repulsing an attack by Conrad's nephew, the future emperor Frederick Barbarossa. The crusaders also suffered a natural disaster, when part of their encampment was swept away by a flash flood with considerable loss of life.
Manuel wished to induce the crusaders to cross to Asia Minor by the Hellespont, keeping them away from Constantinople. However, they ignored the advice of Manuel's ambassador and pushed towards Constantinople, arriving on 10 September. Manuel had extensively repaired and strengthened the walls of his capital as a safeguard against any crusader aggression. The Germans encamped around the suburban palace of Philopatium, but so pillaged it that it became quickly uninhabitable. They then moved to another suburban palace, Pikridion. The crusader force, which may have been suffering from a lack of food, made depredations on, and acts of violence against, the local population. Manuel was determined to get the Germans across the Bosporus as quickly as possible and mobilised part of his military forces to induce them to move.