Starting with the Visigoths in the late 4th century A.D., we have read about various groups of ‘barbarians’ that had greater and lesser effects on the eastern or Byzantine Empire up to the 9th century. So far, which group do you think has had the greatest impact on the Empire and why?
The answer is obvious. The most important barbarian group that had the greatest impact on the Empire is the one that has the greatest impact on our Empire today - Islam and its adherents. Today, we have very few instances of non-Islamic Arabs or Persians, Goths, Franks, Bulgars or Slavs trying to commit mass murder against the Big or the Little Satan or its allies (the United States and Israel respectively).
What the world would look like if Constantine lived during the Mohammedan ecstasy. Would the world continue to regard the Arabian Peninsula as a "Felix Arabia" - a happy backwater rather than the home of a billion man suicide cult? What would the world look like if the Byzantines had acted like true Romans and wiped Islam off the map and plowed salt into Mecca and Medina? What would the world look like if the child-molesting prophet had been strangled in his crib?
The Byzantine Empire stands guilty of the vilest crimes of heresies and treachery. They accommodated the ugly threat to humanity that is Islam so they can stab their kinsmen in the back over the new novel heresy of the time - Aryanism, Monophysitism or the trouble with Icons. The Muslim is at the gate and these Greeks want to melt a portrait of Jesus, really?
History is replete with examples of how Latin rite Christianity would have reacted to the evil of Mohammedanism. Just take a walk down Tours, France, Google the year 1492 in Spanish history or recall these words uttered "Veni, Vidi, Dei Vici." History herself disabused the notion that the Byzantines provided a buffer state between the Infidels and us - instead the Byzantines were a human shield.
The Emperor Alexios I Komnenos tried to reverse near four centuries of tail tucking and skin saving. But his labors wielded half-hearted success because the laborer in the person of Alexios I Komnenos was at best a half-hearted man. To actually compare the Scythians, Visigoths, Ostrogoths, Vandals, Franks, Mongols, Huns or even the Zulus to the Satanic insanity that is Islam is really insulting ancient Barbarians. The Islamic civil war that gave birth to the Shiite and Sunni death cults gave the Byzantines evidence that Islam would not survive had they provided a unified front of self-preservation. Instead, Leo III the Isaurian started wars against his own people for reasons only he can justify - so much for unity.
In conclusion, we still live along Islam's bloody borders because the Byzantine legacy. Not only would our world be different, think if alongside the pedophiles and communists, how depopulated the infernal regions would be. Like I said, the answer is obvious.