The 12th century was marked by a series of wars against the Hungarians and the Serbs. Emperor Manuel Comnenus campaigned successfully in this region, forcing the rebellious Serbs to vassalage (1150-1152) and leading his troops into Hungary. In 1168, a decisive victory near Zemun enabled him to conclude a peace by which Dalmatia and other frontier territories were ceded to him.
For several centuries Constantinople represents both the end of the Roman Empire and the beginning of the Byzantine empire. Meanwhile Rome gradually establishes a new identity - as the seat of the Christian pope.
After the death of Comnenus the Byzantine Empire fell into permanent decline. The poor leaders that followed lost all of the gains that were obtained over the past hundred years. Then, as a result of an unwise taxation increase, a Bulgarian rebellion was organized in 1180, and one year later the lands were lost. These events greatly contributed to the empire's decline.
Byzantium offers the Roman emperor a clear strategic advantage as a centre of operation, for it is much closer than Rome to the threatened regions of the empire.
The main problems in the past century haD been defending the Balkans from invaders beyond the Danube and protecting the Middle East from the Persians. Byzantium, renewed now as Constantinople, sat firmly between these troubled regions.
After Constantinople was sacked in 1204, Nicea, Trebizond and Epirus were named successer states. Under Michael Palaeologus, Nicea was able to recapture Constantinople, defeat Epirus, and reestablish the Byzantine Empire in 1261. Because of a Muslim civil war, the Byantine Empire lasted for a few more centuries. However, under Osman I, the newly founded Ottoman empire was able to overrun most of the Byzantine Empire's territorries. Finally, in 1453, the Ottoman Turks were able to destroy the Byzantine Empire.
Although the heart of Empire was torn out by the capture of Constantinople, the Byzantines themselves show a considerable amount of resilience. Three major "successor states" are set up by Byzantines within the borders of the old Empire. The strongest of the successor states is the so-called Empire of Nicaea. In 1261 the Nicaean Emperor, Michael VIII Palaiologos, succeeds in recapturing Constantinople from the Latins.
Michaelís brilliance as soldier and diplomat restores the Empire to some of its former glory, but he remains an ambivalent figure in Byzantine history - he had murdered his way to the top but had committed a still greater crime in the eyes of his subjects. In the interests of securing some form of western alliance, Michael had attempted forced union of the Catholic and Orthodox churches. Church union is unthinkable to most ordinary Byzantines - their attitude towards the west permanently embittered by the Fourth Crusade.
Here is the Byzantine Empire Timeline:
667 BC- founding of Byzantium
AD 330- Constantinople (Byzantium) becomes Roman capital
AD 395- Empire permanently splits after death of Theodosius
AD 527- Justinian crowned emperor
AD 537- Hagia Sophia is built
AD 554- Justinians generals recover much of the Roman Empire
AD 568- Italy is captured by the Lombards
AD 1014- Basil II defeats Bulgarians at Kleidon, becomes Bulgar Slayer
AD 1018- Bulgaria is conquered
AD 1025- Death of Basil II, beginning of decline
AD 1054- Great Schism
AD 1081- Alexius I arrests decline, joins 1st Crusade
AD 1097- Byzantine and Crusade armies recapture Nicea
AD 1180- Death of Manuel Comnenus restarts decline
AD 1204- Crusaders capture Constantinople and form Latin Empire
AD 1261- Michael Palaeologus recaptures Constantinople
AD 1453- Ottoman Turks capture Constantinople, fall of the Empire