Just in case anybody needs it, here is the history that will go in the OP.
It is the year 634 AD, but events have not transpired in the way they did in our world. Urged on by his mysterious Aksumite wife, the General Stilicho overthrew Emperor Honorius and seized power for himself. A series of strong Germanic Emperors ruled in the 5th century, keeping the Western Empire together at the loss of Britain and Soissons.
Further losses would take place as the rising Barbarian Kings in Francia and Visigothia pushed their territorial rights whenever they saw weakness. When Emperor Remus died in battle against the Visigoths in 598, it looked as if the Western half of the Empire up to the Italian border would be lost in its entirety. A young nephew of Remus, Marcus Marcellus Priscus, was declared Emperor by his soldiers and proved his worth by pushing the Visigoths back in Hispania. He was crowned Emperor Marcellus, and the few feeble opposing claimants were assassinated before he reached Ravenna. When the old ruler of the Eastern Empire died childless, Marcellus moved to take control and proved himself again in a skirmish with the Sasanians. And so the Empire was unified.
The Sasanian threat grew larger when the Shahanshah Bozorgmehr made a shaky alliance with the White Huns. He had expected an exiled Roman prince in his court to be elevated in the Eastern Empire, and he saw the rise of Marcellus as a threat. Confident he could get the Eastern court to support his claimant, the Shahanshah invaded. Off and on wars between the two great powers followed, continuing for nearly thirty years and only ending when Bozorgmehr died. With his other sons lost on the battlefield, the Persian Empire came under control of a three year old boy, the new Shahanshah Mazdak.
In Constantinople, Marcellus split the Empire in two so that his son Flavius Marcellus Pulcher could rule the west and be prepared to control a unified Empire. Pulcher placed his capital in the young port city of Venice. In 630, German incursions through the alps became so intense that both father and son were called to fight on the border.
So when reports of increasing Saracen raids reached the Emperor, he decided to send his second son, Marcus Priscus Caesar, who would prematurely declare himself "Arabicus" before disappearing as his army was destroyed in the deserts of Syria.
Emperor Marcellus and his son, Emperor Pulcher, fight the Germans on the borders of Italy. The second Imperial son, the self-named "Arabicus", has went missing after a defeat, and an eight year old boy named Mazdak sits as Shahanshah of Persia. Blood on both sides have been spilled, and the borders of the two Empires are exhausted from thirty years of hard war. Will they be able to defend against the Saracen threat, or will the world's great civilizations fall to the Arabs and their strange new religion?