So, people tend to confuse the Pax Romana with the period of history during which the Roman Empire existed, but that's not really how it worked.
The late days of the Republic and the period of civil wars that ended with Octavian as Augustus were anything but peaceful. The salad days stretched across most of the first two centuries AD, when transfers of power were mostly peaceful and when they weren't, they were at least brief and disrupted the life of the ordinary people of the empire very little. What wars there were happened out on the frontiers and Rome was generally victorious. The rule of law more or less functioned properly, and the aqueducts ran on time.
Omens of future problems came with the reign of emperor Joaquin Phoenix who (after going crazy and arranging for the murders of a bunch of senators) was not actually stabbed in the Coliseum by Russel Crowe, but rather was strangled in the tub by his wrestling coach and personal trainer.
There was a brief period of semi-stability under the increasingly silly and neurotic Severan Dynasty before the wheels came off in the Year of the Six Emperors, which kicked off the Crisis of the Third Century, a period when two centuries' worth of chickens came home to roost in Rome.
The crisis began when Maximinus Thrax, a "Barracks Emperor" who completely lived up to his anime villain name, got folks fed up. Some younger, well-to-do Romans in North Africa stabbed the local tax collector to death and talked the provincial governor into declaring himself emperor. Since the dude was an octogenarian, he nominated his son as his co-augustus.
Unfortunately the governor of the next province over not only remained loyal, but was a better general. The rebel army got crushed in the field at Carthage, killing the son (Gordian II), and on hearing the news, the dad pulled an Aaron Hernandez to avoid capture and execution.
On this date in 238AD, the senate then appointed a couple of elderly senators, Pupienus and Balbinus, with good military histories and prominent committee memberships (most importantly, both were ranking members of the What the Hell Do We Do About Emperor Maximinus Thrax Committee) as co-emperors.
This proved about as popular with the general populace of Rome as would the Senate suddenly appointing John McCain and Lindsey Graham as co-presidents to unseat Trump. The PR problem became apparent when the new co-emperors couldn't appear in public without people throwing stuff at them, and so the senate named the Justin Bieber-looking 13-year-old grandson of the recently-hanged Gordian I as Gordian III.
Anybody with a room temperature IQ could see that Balbinus and Pupienus were still in charge and the barely-pubescent Gordian was a figurehead, but the populace of Rome was mollified by this move, which doesn't speak much for their collective smarts.
Meanwhile, Maximinus Thrax was making his way from his home base in the Balkans toward Rome. He arrived at the city of Aquilea but, rather than welcoming him and resupplying his troops, the city shut its gates and forced a siege. With the senatorial army led by Pupienus closing in from Rome and supplies growing short, Maximinus Thrax wound up getting shanked by his own troops, along with all his family and staff.
With the guy with the anime villain name dead, McCain and Graham naturally started quarreling and plotting against each other. Before it could come to open war, the Praetorian guard killed them both and left Justin Bieber lookalike Gordian III sole emperor at the end of the year.