The Byzantines continued the Roman tradition of using mercenaries in their army, and by incorporating both light cavalry and heavy infantry from each new group that attacked them they kept it a redoubtable fighting force. The Varangian Guard was a regiment of Scandinavians, originally. It began with 6,000 Rus sent by Vladimir to Basil II in 988 as an inducement to give him his sister's hand in marriage. Basil appreciated their fighting abilities and trusted their loyalty (bought with much gold) over the native Byzantine guardsmen, so he promoted them to the position of imperial bodyguards and their commander was given the title of Akolouthos, "acolyte/follower." Byzantine writers talk at length about their loyalty and fighting ability; they primarily fought with long axes and "were frightening both in appearance and in equipment, they attacked with reckless rage and neither cared about losing blood nor their wounds," which may suggest berserkers. Their loyalty was to the throne; in 969 Nikephoros II was assassinated - a servant managed to call for the guards but when they arrived he was dead and they immediately knelt before the murderer, John Tzimiskes, and hailed him as Emperor. "Alive they would have defended him to the last breath: dead there was no point in avenging him. They had a new master now." One of the perks of Guard membership that ensured this loyalty was that when the Emperor died, the Varangians as his personal bodyguards had the right to plunder as much gold and jewelry from the imperial treasury as they could carry, a procedure known in Old Norse as polutasvarf, "palace pillaging." They were also famous for their drinking; in 1103 King Eric the Good of Denmark, visiting Constantinople, saw fit to exhort them to "lead more sober lives."

"Varangian" was used in Slav and Greek to refer to all Scandinavians; Old Norse Væringjar means "confederates." The guard continued to draw from all over Scandinavia until the late 11th century. In Sweden, so many were leaving to enlist that a medieval law forbade claiming an inheritance while still in "Greece." But after 1066, the Varangian Guard came to include more and more Anglo-Saxons dispossessed by the Normans; in 1088 a large number of Anglo-Saxons and Danes emigrated to the Byzantine Empire via the Mediterranean (one source says more than 5,000, in 235 ships), many of whom joined the Guard, which was from then on usually called the Englinbarrangoi, "Anglo-Varangians." The Guard remained an elite fighting force and is thought to have been disbanded after Constantinople was sacked in the Fourth Crusade in 1204, because they were the most effective force among the defenders and had successfully repelled the initial assaults, although by some accounts it was still active in the 14th century.

The most famous member of the Varangian Guard was Harald Hardrada, who rose to become its commander and used it as his private army to conduct lucrative raiding campaigns in North Africa, Syria, and Sicily. He also served through the deaths of three emperors and thus got to plunder the treasury three times. He left the Guard in 1042 - without leave from the Emperor - and returned to Kievan Rus, where Yaroslav finally let him marry his daughter Elisabeth (Ellisif in ON sources) before he returned to Norway in 1045 to become king.

Note: Wikipedia (link will open a new window) and its linked articles contain a wealth of information on where the Scandinavian members of the Varangian Guard and the Rus came from, showing the preponderance of Upplanders.