Marcus Aurelius is often held up as an example of a great emperor, the last in the Golden Age of Imperial Rome which began with Nerva and included Trajan, Hadrian and Antoninus Pius. Similarly, Basil II was perhaps the single most forceful emperor in Byzantine history and, after an initial defeat, accomplished innumerable military successes.
Yet both were responsible in large part for the decline and ultimate fall of their respective empires.
Succession is a critical issue in the ancient and medieval worlds. It wasn’t a problem unique to the Roman Empires. Caliphates and Sultanates often descended into brief civil wars over succession as well.
The Golden Age saw emperor after emperor nominate his own successor. During this prolonged period of prosperity for the Empire men of worth were promoted, and the peace (for Rome, at least) enabled a strong class of loyal and successful men to build up.
Marcus Aurelius buggered it up on a permanent basis. His first co-emperor was Lucius Verus. Verus wasn’t especially bad, a bit of a boozy fellow but not a vicious lunatic. Verus died, and Marcus Aurelius then named as his successor his son, Commodus (made famous by the excellent film Gladiator). The Commodus of reality was not that different to that of the film (probably a bit bloodthirstier and more competent as a warrior, actually). He killed a significant number of senators who should have been serving the Empire, and was so bad he ended up being assassinated, and replaced by a short-lived successor who was toppled by the Praetorian Guard.
From that point on the imperial seat became the plaything of the army, to a greater or lesser extent. No period of imperial adoption recurred, and the Empire began a steady spiral of decline, occasionally delayed and only once truly reversed (with the excellent Danubian general-emperors such as the Gothic Claudius and Aurelian), but that was a brief respite.
Basil II’s case is a little odder. He had been officially emperor for a long time before he really took on the job, as a number of successful generals seized the throne but also shared it with him (they ran the Byzantine Empire but, slightly unusually, did nothing to harm the ‘official’ imperial family, which was Basil and his younger brother Constantine).
Basil never married or had any children. He may have been gay or simply disinterested (whilst unusual, something like 1% of people are asexual). His brother was technically co-emperor but was happy to spend his time in luxury whilst Basil actually ran things, and he also had several nieces to guarantee the family line would continue (worth pointing out the Macedonian Dynasty, of which he was a member, had been going for over a century at this point).
It would have been better if he had had children, and his brother had not. When Basil died, his brother took the reins and proved seriously inept. The shortness of his reign did limit the damage he could do, but Basil’s niece, Zoe, ended up having various marriages to those who aspired to the throne, and they tended to be bloody awful.
Byzantium underwent incompetence and turmoil (with a brief respite for Isaac Comnenus who sadly reigned only two years) until Alexius Comnenus (Isaac’s nephew) came to power. By then, the Empire was in bad shape. Assailed by the Norman adventurer Robert Guiscard (who destroyed forever the Byzantine presence in Italy), having to cope with the First Crusade (which proved at least less damaging than the Fourth...) and struggling to regain the Anatolian territory which had formed the heart of the Empire’s manpower, Alexius and his two successors performed admirably.
But the years between Basil II and Alexius had taken a permanent toll. Venice had dominion over the seas, Italy was lost forever, and the Turks were getting ever closer to the city itself.
Basil II, as emperor, was almost an unmitigated success. He utterly dominated the Empire, crushed his enemies and was absolutely devoted to his army, which he transformed from the dregs of civilisation to the most formidable force in the world.
But when it came to the succession he failed. His brother was clearly disinterested, and he had no nephew to take the reins directly (instead Zoe was used to assume power repeatedly).
Both Marcus Aurelius and Basil II are often considered amongst the finest of emperors. I find the former particularly overrated, for Basil’s error was perhaps more explicable (his brother would continue a long-running and successful dynasty whereas Marcus Aurelius ended the adoptive approach in favour of an incestuous psychotic) and he was more impressive as an emperor. But it goes to show that whilst the lack of an heir could lead to chaos, dissent and civil war, the presence of incompetent successors was even worse.