Marcus Aurelius would probably have been remembered as a philosopher even if he had not gone into politics, a unique achievement.
At an early age, long before becoming the Emperor was even a remote possibility, Marcus embraced the philosophy of the stoics. It seems that from that time on he was first and foremost a philosopher and behaved accordingly even after he came to the throne. His Meditations, written in the winter campaigns on the Danube, are still popular today. Amazon offers several pages of differing editions of his work, all of which are frequently reviewed and which get almost universally positive ratings. He is quoted roughly once an hour on Twitter.
The Meditations are well worth a read. You can read them simply as an historical document, though there isn't much in the way of actual direct historical information in them. They do give you some insight into the way the Romans lived and thought. They are certainly interesting as a portrait of the personality of the man himself, and he comes across very well in them. But most remarkably, there is also a lot of straight forward wisdom and understanding that makes them a thought provoking and interesting read in their own right regardless of the historical background. These days the self help sections of bookshops grown under the weight of advice on how to live our lives better. Few are better than the Meditations.
It is a little ironic that the most philosophical of the 5 good emperors should also have been the one most troubled by warfare, but it was probably a sign that the world was becoming a more dangerous place. Across Asia tribes were on the move threatening the Empire's security. The philosopher emperor, true to his stoic values, did not love war. But he was prepared to do his duty and fight off aggressive incursions by personally leading the army if necessary. There was a war against the Parthians followed by a revolt by the successful general, Avidius Cassius.
This revolt may have been a misunderstanding following a false report of the death of Marcus. It certainly gave Marcus the opportunity to show off his stoic philosophy. As Gibbon notes - he regretted that the rebel's suicide robbed him of the chance to experience the pleasure of turning an enemy into a friend. Marcus had no military experience prior to becoming emperor but seems to have been a good enough general. He drove back German incursions into Gaul successfully.
A more serious attack on the Danube required 9 years of continuous campaigning, including fighting in the winter. Unusually harsh winters caused the Danube to freeze over giving barbarians the opportunity to avoid the frontier's fortifications. It was during this campaign that he composed his Meditations, and it may well have been the stress of this campaign that killed him. But at this point the empire was clearly still very strong militarily and able to cope with blows from seemingly all directions. From this time on, things will only get worse.
The greatness of Rome in history is probably more due to this period of benign dictatorship than to anything else. The emperors themselves were capable if not entirely likable and overall they kept their huge state in good order. A benign dictatorship, but a dictatorship nonetheless. For all the outward form of the republic there was no redress against the power of the state. It was very much the forerunner of the modern totalitarian states we know from recent history. But there is one way in which the Empire of Rome differed, and differed significantly from modern tyrannies. Its sheer size made it impossible to escape from. There were no asylum seekers in the ancient world: there was no asylum. As Gibbon puts it:
'But the empire of the Romans filled the world, and when the empire fell into the hands of a single person, the world became a safe and dreary prison for his enemies. The slave of Imperial despotism, whether he was condemned to drag his gilded chain in Rome and the senate, or to were out a life of exile on the barren rock of Seriphus, or the frozen bank of the Danube, expected his fate in silent despair. To resist was fatal, and it was impossible to fly. On every side he was encompassed with a vast extent of sea and land, which he could never hope to traverse without being discovered, seized, and restored to his irritated master. Beyond the frontiers, his anxious view could discover nothing, except the ocean, inhospitable deserts, hostile tribes of barbarians, of fierce manners and unknown language, or dependent kings, who would gladly purchase the emperor's protection by the sacrifice of an obnoxious fugitive. "Wherever you are," said Cicero to the exiled Marcellus, "remember that you are equally within the power of the conqueror."
Follow the next episode in this extended review of Gibbon's Decline and Fall of the Roman Empire: Commodus