The assertion of authority by the Senate was to prove short lived. Even while Maximus had been in the field against Maximin, there had been trouble in Rome between the Praetorians and the people, and blood had been shed. The rule of Maximus and Balbinus was soon brought to an end by the now standard procedure of the soldiers simply killing them. This happened only a few months after the triumphant return of Maximus.
It is unlikely that anything could have saved the Senatorial candidates, but their removal was made extremely easy by the availability of Gordian the third. An easy to manipulate child was easy prey. Gordian, about whom we know very little, was hidden away with a crowd of eunuchs and told nothing about what was going on in his empire. What was going on was of course flagrant abuse of power by the soldiers.
Towards the end of his teenage years he managed to break out of the hold the eunuchs had over him, get married and appoint his father in law as his main minister and crucially as the Prefect of the Praetorian Guard. Gibbon interprets this as all being a positive move and portrays the father in law, Mesitheus, as a noble man only concerned with the good of the empire and the emperor. The record of his achievements is certainly sound. He strengthened the defences in Africa and led a successful campaign against the Persians. The Persians had invaded the empire and gained control of a couple of fortresses. Mesitheus drove them back. He was planning further campaigns against them when he died suddenly - a bit suspicious that. Gordian appointed Philip the Arab to replace him.
This may have been a mistake. Philip was according to Gibbon an Arab, and therefore a former professional robber. This is a rare example of Gibbon getting things wrong by a considerable margin. For a start, Philip was almost certainly a well assimilated person of Arab origin rather than some adventurer who had just come in off the desert. It was also somewhat unfair even in Gibbon's day to characterise Arabs as robbers. Nomadic people have a different set of values to sedentary ones, and don't regard property rights in the same way. It was not really fair to categorise a whole race as robbers.
But Philip certainly was ambitious and able, and quickly replaced Gordian as emperor. This was probably achieved by the simple expedient of killing him. Some sources say he contrived a shortage of provisions to stir the troops up against the emperor. Gibbon gives a detailed account of the murder of the young man, complete with details of his pleading for his life and a show of some compassion on the part of Philip. He nearly spared him, but then reflected that given his popularity it would be unsafe to allow him to live as a possible focus for opposition. Gibbon is aware that the account he gives is suspect, and warns the reader. A Persian account unavailable to Gibbon has Gordian being killed in battle. This seems on the whole more likely and gives Philip a better character.
In any case, Philip returned to Rome to get his position ratified by the Senate, so he was at least keen to appear legitimate.
There happened to be a highly convenient occasion just on cue. Three years after Philip came to the throne, Rome celebrated 1000 years since its foundation. This took place in 248. (Rome was traditionally founded in 753 BC, I have never worked out how the maths is supposed to work on that either). To mark this auspicious millennium the Secular Games where celebrated. These had been ostensibly revived by Augustus (probably invented) and were very infrequent events. They would only occur once in a person's lifetime. Despite the name, there was a strong religious component to them. There was also a strong national element. Slaves and foreigners were not admitted. Philip's, appropriately, were particularly spectacular.
Gibbon reflects on the state of the empire. It was still, he noted, as large as it was in the reign of Hadrian. But the appearance was deceptive. There was nothing left of the rugged farmers who could don swords and armour to conquer the world and return to their fields to pick up the plough just where they had left it. Power had become concentrated in fewer and fewer hands during the time of the republic. For a while it had been wielded by the emperor alone, but still with some semblance of attention to the general well being. But now, despotic power was wielded by whoever could get control of the military machine. And after Maximin's elevation, there was no reason any man could not in all seriousness aspire to rule if he pursued his goal with sufficient energy and ruthlessness. All that had to be taken care of was ensuring that the mercenary troops got paid enough to ensure their loyalty.
Over time, this eroded the productive activities of the empire. The industry and commerce that flourished under the five good emperors was wrecked by continual political instability and rapacious tax collecting. Its borders were, for now. secure. But both inside and outside the empire, signs of decline were becoming apparent to anyone who chose to look.