The Siege of Jerusalem was an armed conflict between the Crusader Armies and the Sarecen Army. Saladin, still smarting from his defeat at Hattin, ordered an advance of Jerusalem on September 15, 1187. King Guy was elsewhere, at Acre, so the Saracens took the city relatively easily.
Later that month, Crusader soldiers under the command of Prince Richard of England surrounded the city. During the first few months, the Saracens were able to hold off the Christians with Greek fire, a napalm-like substance. But, as the summer of 1188 began, both sides were suffering heavy casualties. The Crusaders were planning on returning home when Barbarosa arrived.
Barbarossa, the German king, had thus far stayed out of conflict in the Middle east, because his neighbor, the Byzantine Empire, would not let him pass. After taking Constantinople and crowning himself Byzantine Emperor on January 2nd, 1188, Barbarossa's giant army moved across the Bosphorus and marched to Jerusalem.
With the German reinforcements, the Crusaders broke through Sarecen defenses and entered the city on August 20, 1188. Saladin and his forces surrendered later that day. Saladin exiled himself to Baghdad, where he died bitterly two years later.
King Guy, who at first was overjoyed by the victory, grew suspicious when Crusader families from Europe came into his lands. After questioning Prince Richard on why they were there, Richard threw Guy into jail and proclaimed himself Emperor of the Holy Land.