OT : I will multiply all the city and unit numbers by 10 for this AAR. This will make the numbers more realistic in my opinion, for example Rome would have about half a million citizens instead of 50.000 for example, and battles could have over 20.000 soldiers instead of just a few thousand.
While the senate was not loved, it had at this point ruled Rome for over 100 years, and the tales of what Vonones had done did little to make the people trust him. The governors in Hispania, all of Africa, half of Gaul, Britannia, Greece, Dalmatia, Dacia, Moesia, Pannonia, Galatia, Commagene and southern Italy decided to fight him. It took some time for the east to be able to muster, but in 294 the other provinces were fully in control of forces loyal to Vonones, and in 295 Galatia and Commagene were overrun. The Greek army that had marched on Thracia turned around after word came there that Vonones had captured the fleet at Ostia. In Moesia the Governors formed a joint army with 25.000 men that marched towards Thracia and the east.
Meanwhile Vonones had subdued the rest of the Italian peninsula and the islands close to it. In Africa General Esdras also declared himself emperor, and began a campaign to conquer Numidia and Mauretania.
Vonones was forced to leave large forces behind in the areas that he conquered, Italy had still been spared the worst parts of the senates bungling, and the people were still loyal to the republic. The Sassanids tried to use the situation to their advantage, and they invaded the east in 293. The Roman soldiers in the area were well trained, but they lacked real leadership, and the war became a stalemate with both sides sometimes taking a city, sometimes losing it. In any case, the east was unable to send any large forces to the west, but the city of Byzantium just west of the Bosporus was supplied by sea with food and men and the senatorial forces were unable to take it.
In Germania the tribes had been well beaten, and were unable to launch anything more than small raids, but in the south the Berbers were able to do havoc in Mauretania. In 295 Emperor-General Esdras moved a large force west to capture it after having taken most of Numidia the two previous years.
In Italy, Vonones had formed an army with 20.000 men, and decided to send it to the south to capture Africa. The harvest in Italy had been poor in 294, and he needed the food to make the Italians love him. The campaign went well, in 296 Africa was his, and the next spring he subdued Numidia as well. Esdras had met a large Berber force somewhere in the desert, and had been killed. Vonones decided that Mauretania would not be worth the fight, especially as senatorial armies had finally taken over the Germanian provinces and were currently besieging Mediolanum.
Over the next 10 years he fought several battles in Gaul, Germania and Hispania, finally conquering each of them. Revolts in recently captured cities were common, but finally each of them fell. In 308 he invaded Britannia, and conquered it in 310. In 311 he began his campaign to take over everything up to Byzantium, Dalmatia, Greece and the northeast were still very much controlled by republican forces.
The senatorial defense of these provinces was poor. They lost every battle they went into, they had little food to survive sieges, and the commands to the armies were lost or sent to the wrong places. In 314 the Goths moved into Dacia, defeated the senatorial forces there and began to settle. In 316 Vonones tried to retake Dacia from them, but their heavy cavalry managed to break Vonones lines at two key battles, and he was forced to pull back and sign a treaty with the Goths giving them Dacia.
At the sight of this a senatorial army tried to march north from Thessaloniki to defeat the remaining forces of Vonones. While the senatorial army was over twice the size of Vonones, he was still able to defeat it. It took a few years however for Vonones to be able to march against Greece, and the last senatorial fort there fell only in 321 on Crete.
He had seen during the war how big and ungovernable the Empire was, and decided to split it into two parts, a western and eastern part. It took some time for him to plan it, but in 325 the final borders of the parts were decided. The capital of the eastern empire would be in Byzantium, the western capital in Massila, and the capital of the whole Empire in Rome. The elder son, Valentinianus, became leader, or Augustus, of the western part, while the younger son Valens became Augustus of the eastern part.
There were 12 years of peace, until a new crisis broke out in 337 when Vonones, Valentinianus, Valens and several other family members converted to Christianity. At first there was little uproar, the old Roman gods had slowly by constantly fallen away from use, and the Empire was filled with new and interesting religions. But when the Emperors declared Christianity a state religion in 339 and forbad worship of other gods in 343, a new wave of rebellions sprung up. It did little to help that Vonones died early in 344, and the two brothers were unable to agree on who should be the new emperor. Instead they decided that the Roman Empire should from now on have two emperors, Augusti, each with a heir and sub-emperor called Caesar to aid him.
In the west, the initial wave of religious violence lasted until 350, and Valentinianus became known as the Wrathful. Whenever he saw a pagan, a non-Christian, he would attack the man with his bare fists, often not stopping until the man was dead or had converted. Valentinianus had otherwise a bad temper too, but he was still a good politician and managed to bribe his enemies off and keep the cities in order. Unfortunately the wealth of the Western Empire was sinking all the time, and while he did try to keep good and well trained units in the cities, they were very few everywhere in the cities but in the Germanian border.
He did his best for as long as he could, but in 363 it was finally at an end, the state coffers would not support the army for one year longer unless something drastic was made.