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By 636, the Arabs had overrun Egypt, and in 637, they attacked the Rump Empire of Eastern Rome. In less than three months, they overran it, and they were on the straits of Sicily, calling for Shahrbaraz to send his warships. He was surprised; he had not expected them to take all those lands that quickly; the Arabs were advancing far too fast for his tastes. He sent a reply saying that his ships would be slightly delayed in coming, but that the Arabs should be ready to leave at any moment.
For his own part, he mobilized his armies, and assembled a force of thirty thousand, and with five hundred warships as well. He sent word to the Avars, and convinced them to attack the Lombardians in Northern Italy, and then boarded his ships and sent his fleet across to attack the port of Bari, which they did easily. Meanwhile, his ships reached Carthage, and ferried the Arabs across to Sicily, which they subdued in a months time, and then onto the boot of Italy. At the same time, the Avar tribes took Milan and Pavia from the Lombardians easily, though the then tiny city of Venice resisted their attempts to take it.
The Arabs continued their advance up the Italian Peninsula, and they linked up with his army at Naples, which, after a short siege, fell to the combined Roman and Arab forces, even as the Avars took Pisae, Ravenna, Firenze, and advanced on Rome from the north. Their armies united at the walls of Rome, and put the city under siege. The Pope attempted to flee down the river to get a ship into exile, perhaps to the Christian kingdom of the Franks, but was captured by Roman ships and taken prisoner.
With their leader gone, and a promise of safety to the people of the city if they were to give up peacefully, Rome surrendered only three days after the Pope left the city, and thus in 638, the Roman Empire was once again rulers over Rome, even if it was a Persian dynasty. The Arabs were given the half of the citys treasury, as they were promised, and also a large fleet that had been captured from the various Italian cities, with which they advanced across the Western Mediterranean, attacking Sardinia, Corsica, the Balearic Isles, and the Berber kingdoms of the Maghreb in 639-640.
Shahrbaraz offered the Avars extensive lands in Northern Italy, but when word reached them of the fact that they were being attacked by the Khazars, they refused the claims and marched home to defend their people. Thus, Shahrbaraz spent most of 639 subduing the last few recalcitrant garrisons of Italy, like Genua and Venice, when word reached him late that year that there was a rebellion in progress against the Persians in Greece and Albania.
In 640, Shahrbaraz led his last campaign, destroying the Grecian rebels, and securing the rule of the Parvez Dynasty over the Roman Empire. Late that year, he died, leaving his son, who would be known to his subjects as Cavadius I Parvez, to rule the Roman Empire.
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The 640s and 650s were turbulent decades, with new leaders coming to power all over the world.
Most striking of the changes was the already mentioned fact that Shahrbaraz was replaced by his son, Cavadius. Cavadius had been groomed as an heir as much as possible, but long periods away from Constantinople meant that Cavadius tended more towards the pleasures of the flesh than his father, and the first few days of his reign were marked by a large amount of alcohol induced vomiting.
Of course, he sobered up relatively quickly, but his drinking problems were to plague him for the rest of his life. One of his first acts as king was to send a new embassy to the court of the Caliphate, and reaffirm their alliance; he had realized that much needed to be done. Together, the Arabs and the Romans soon planned a new strike into the heart of the remnants of the Sassinid Empire, however, this was delayed by the assassination of the Caliph Umar in 644.
Uthman succeeded Umar without much dispute, but he asked Cavadius to postpone the invasion plans for at least a year while he cleared up some difficulties with the supporters of Ali, who were disputing the Caliphate.
Meanwhile, Arab forces cleared up the last of Berber resistance to their rule in Northern Africa, and one of the local generals was slowly uniting their forces into one, to create a force which could invade and destroy the Visigothic Empire on the other side of the Pillars of Hercules. The Visigoths, for their part, were busy fighting the Franks in Gaul; the Khazars were still dueling the Avars to the death, and Cavadius for conquered Armenia and Georgia.
Finally, in 646, the Arabs and the Romans launched a joint assault on the last gasp of the Sassinian Empire; it was overrun easily, the Romans washing their boots in the Caspian Sea (figuratively, of course, as it was too salty for any sane person to want to do that), the Arabs advancing deeper and deeper into Asia.
Now, though, the Roman Empire had met an interesting position. They could not expand into the Slavic lands, for there were their allies of the Khazars. They could not attack East, for there were the Arabs. West were the Visigoths, a land promised to the Arabs, and the Franks, an Empire strong enough to prevent most attacks. North lay the fierce Germanic tribes. And thus, their conquests met an impasse, or so it seemed...
However, rumors from the north had it that a new realm was rising. A realm of Slavs, led by a great Frankish general, who had defeated Frankish invasion forces. Samo, he was called, and he ruled over a realm by the name of Greater Moravia. Soon, Cavadius got it into his head that the conquest of this realm would lead to rule over all of Germania, and he ordered a northward expedition to take this realm.
The initial expedition was a broad front advance that secured all of the land to the Danube river for the Roman Empire. Next, an army of thirty thousand marched northward, to meet Samo on the field of battle.
The Roman armies encountered the armies of Samo just after crossing the Danube River, and drew up for battle on a cold, rainy, windy day. The wind was coming from the north, so it blew directly in the faces of the Roman armies, making visibility difficult; but Cavadius ordered the advance anyway, his infantry in the front, lancers on the flanks, but his cataphracts and Avar mercenaries were kept to the rear.
Samos forces were drawn up arrayed against them, on the opposite side of the field. His men had little armor, inferior weapons, and poor training, but they were well motivated and led, and they were fighting for their homeland; their morale was far superior to the Roman army. His men went forward, charging at the Romans.
Both armies were running now, slipping and sliding over the wet grass, as the rain began to ease up, and fog started to roll over the valley. The initial clash seemed to favor the Slavic hordes of Samo, as the pressure of rank upon rank of screaming Slavic warriors smashed into the line of Roman infantry. Then Cavadius sent half his cataphracts around his right to attack Samos left, and there they met a line of armored spearmen. Another flanking attempt by his Avar mercenaries was met with a cavalry charge from the Slavs, and the line seemed at an impasse, with the Romans being slowly forced back.
Samo personally led a charge into the center of the Roman line with his cavalry reserve, but it was met by the remainder of the cataphracts, and Cavadius rallied his infantry. The battles tide suddenly turned, the middle of the Slavic line folding, and then the Romans drove the Slavs back, back, and still further back, routing their army.
Cavadius wanted to exploit this victory by conquering the kingdom of the Slavs outright, but his more moderate advisors convinced him that they could serve as a valuable ally in the northern Germanic region, and that with the Slavs as allies, he could then sweep over to the Rheine and beyond, taking the Franks in a grand flanking maneuver, and with them, Gaul.
He agreed, peace was signed with Samo, and a new army, this one forty thousand strong, was assembled, and directed against the Franks.
The Franks at this time were divided into three kingdoms, and the one that the Romans marched against was Austrasia, a large realm ruled by the kind Sigebert III. Sigebert was most famous for being the first do-nothing king of the Franks (well, thats OTL. Im not telling wether there will be any more of them, nor wether there will indeed be any Frankish kings period). He was termed as such because he was not nearly as influential in his own kingdom as Grimoald the Elder, Mayor of the Palace, his chief advisor, friend, and the real power in the kingdom.
Grimoald recognized immediately that the Romans were the greatest threat to the kingdom of the Franks in all of their as yet rather short history, and appealed for help to the king of Neustria (the other major Frankish kingdom), Clovis II. Clovis II was, however, under the influence of the secular magnates of his kingdom, a mere boy, and the magnates, who wished only power for themselves, were hesitant to lend too much aid, as that would siphon away power that would be needed against the Visigoths. Thus, they sent only a small expeditionary force, a mere four thousand men.
It was not nearly enough. The Austrasian kingdom was already desperate for men, and Grimoald left no stone unturned as he assembled his army. In the end, he managed to get together a force of about twenty thousand, but even with the expeditionary force from Neustria, it was still vastly outnumbered by the Romans.
Thus, he created a diversion.
He sent a force of five thousand men into Italy, attacking, raiding and pillaging cities all over the Po River Valley. The Romans honor was insulted by this, so naturally Cavadius sent a force, of about 10,000 Romans, to meet the enemy and bring them to battle. The combined Roman invasion force, then was 30,000, still larger than the Franks, but instead of two to one, it was now more like three to two in favor of the Romans... Furthermore, Grimoald was able to draw off even more forces of Samo to fight useless skirmishes to the north of his territory.
Thus, the battle was joined, with twenty thousand Roman and Moravians meeting a similar number of Franks. This battle would decide the future of Austrasia; if the Franks lost, it would inevitably fall into the hands of the Romans; if they won, they would repel the Romans from Germany, and probably capture their emperor far from home...
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