The Byzantine Empire suffered a major setback at Manzikert in 1071. The defeat cut the empire off from its traditional military recruiting grounds. Over a century later, the empire had not recovered. On top of this, tensions with Western Europe over religious differences and the death of the emperor further weakened the Byzantines. Finally, the Normans attacked and conquered the major Byzantine city Thessalonica in 1185. The defeat proved a major setback further weakening the empire and eventually leading to a collapse.
By 1180, the Byzantine Empire seemed as strong as ever. However, the Muslims pushed the empire from the east and the Latins from the west. The Byzantine military was substantially weaker by the late twelfth century than just 100 years before. The defeat at Manzikert severed the empire from able bodied soldiers.
With Islam pressing the empire, the Byzantines should have expected help from their Christian brethren. However, the eastern and western churches broke over theology, doctrine, the use of leavened or unleavened bread in the Eucharist, and papal authority. The great schism occurred in 1054 and never healed. As a result, the Byzantines found themselves surrounded by hostile Muslims and Latins.
The hostility was reciprocated by the Byzantines. Emperor Manuel I Komnenos died in 1180 leaving an incompetent successor. Two years later, the Byzantines massacred thousands of Latins living within Constantinople. This opened the door to western reprisals. In 1185, the Normans, descendants of the Vikings, decided to eliminate the Byzantine Empire and conquer Constantinople.
The Normans failed to take the walled city, but did succeed in taking another major prize. Sicily's King William II the Good gathered a fleet of 200 ships and an 80,000 man army for the task. They marched through Albania and Macedonia to Thessalonica's walls. On August 15, the Norman fleet arrived and began the siege of the Byzantine Empire's second largest city.
Four other cities had fallen to the Normans en route to Thessalonica. The invaders had high hopes and perhaps delusions of grandeur. They also had an unwitting ally in the city's governor. David Komnenos failed to prepare for a siege. His incompetence led to disaster. In fact, he proved so unfit for the challenge, that some wondered if he wanted the city to fall.Komnenos was not alone in his incompetence. The Byzantines sent two armies to relieve the city. However, only two units arrived at Thessalonica. The army commanders refused to coordinate dooming the city. As a result, the Normans breached the walls and the city fell on August 24. Savage fighting occurred in the streets before the fall. The Normans massacred over 7,000 citizens in the frenzy. They lost 3,000 men in the battle.
In the aftermath, Isaac II Angelos staged a coup. Meanwhile, a Byzantine mob ripped Emperor Andronikos I Komnenos to pieces. The Byzantine army regrouped to defeat the Latins at Dimitritsi. This exposed the Norman position at Thessalonica. In response, the Normans fled the city when it became untenable. The assault, conflict, and massacre deepened the gap between east and west. In 1204, a large Crusader force arrived at Constantinople on its way to the Holy Land. The lure of vengeance for the Massacre of the Latins, memories of Thessalonica, the Great Schism, and the lure of easy plunder transformed the Crusader mission. They reoriented their effort from the capture of Jerusalem to the sack of Constantinople.
Relations between the Greek Orthodox and Catholic Churches reached its nadir in the late 12th and early 13th centuries. The Massacre of the Latins provoked a Norman invasion. The Normans hoped to capture Constantinople, but settled for the Byzantine Empire's second city. The westerners plundered the city and massacred the citizens. The Norman victory led to a Byzantine coup, but also may have inspired a Crusader assault on Constantinople itself in 1204.