As the western half of the Roman Empire collapsed, the eastern part centred in Constantinople carried on (sometimes referred to as the Byzantine Empire).

Often subjected to raids, not just from Vikings, the Byzantine Emperors had amassed an army to protect the empire. Various treaties had been signed between the Vikings and the emperors to secure a truce which provided for Vikings to serve in the Byzantine army.

In 988 CE Emperor Basil II requested assistance from Vladimir I of the Kievan Rus to help defend his throne. Vladimir sent around 6,000 warriors; this is believed to form the start of the Varangian Guard, the name given to the Byzantine army.

Harald Hardrada, the famous viking of 1066 fame, served in the Varangian Guard before becoming king of Norway in 1046.