Great question, I’m looking forward to the other responses that I’m confident are in the works! I’ll talk a little about what I know, the survival of the Roman Empire through what is often referred to as the ‘Third Century Crisis’. There is ongoing debate on what caused the Crisis and what were the actual serious afflictions that weakened the Empire. Some historians downplay the severity of the period, and others point out that it would be more correct to refer to the period as the ‘Crises’, as from 235 CE the Empire faced civil war, foreign invasion, economic crisis, disease, banditry, climate change, and more.

The source material is particularly fragmented for this period, and the exact sequence of events is often unclear. The narrative histories of the period, including Zosimus, Eutropius, and the Historia Augusta, are often woefully inadequate. What we know is that in 235, the emperor Alexander Severus was assassinated by his own troops, and an officer by the name Maximinus Thrax took over. This sparked a series of civil wars as different power blocs in the Empire competed for control. These civil wars often involved marching on Rome itself to be proclaimed as emperor by the Senate there, or to overthrow and kill an emperor reigning from the imperial capital. This of course pulled the armies away from defending the frontiers, which (although often porous in terms of economic and cultural exchange) were intended to be ‘hard’ borders which no enemy could cross. Seeing an opportunity to gain great wealth and political capital amongst their people, non-Roman groups in northern Europe launched huge raids south across the Rhine and Danube Rivers and across the Black Sea into mostly undefended Roman territories, all the while claimants to the position of emperor distracted and weakened each other with incessant civil wars and overthrows. Although the Roman armies were still effective, there was minimal lasting central control and basically zero sovereignty over any territory were an emperor and his army was not present.

Combining with the other afflictions listed above, the outlook seemed particularly dire when in 260 emperor Valerian was captured after a losing battle and imprisoned by the Persians, and the provinces of Gaul, Britain, and Hispania broke away from central Roman control and formed what we now call the Gallic Empire (they continued to call themselves the Roman Empire, but historians love to assign new names to things). Valerian’s son, Gallienus, continued to face civil war and foreign invasion across the Danube River, allowing the Persians to launching devastating attacks and seize Rome’s eastern territories. This led to the city of Palmyra, previously a Roman ally, forming its own increasingly independent kingdom in the Levant and surrounding areas from 267 onwards. I think it is pretty safe to say that not only was the Empire in serious decline at this point in time, but it had catastrophically fragmented. But, as they say, the night is darkest before the dawn…

First amongst the reasons why the Roman Empire was able to cover, in my opinion, is the tenacity and vision of its leaders. Whereas many of Rome’s weakest periods, including the inception of the Third Century Crisis, were characterized by weak, impotent, and incapable emperors, its periods of recovery often featured military tested emperors who vigorously asserted the power of Rome’s armies. Gallienus was killed by his own people in 268, and Marcus Aurelius Claudius succeeded. A career soldier, he is often known by his honorary title ‘Gothicus’, awarded for his major victory over the Goths in 268/69. For defeating the Alemanni only a few months later, he was also awarded the title ‘Germanicus Maximus’. He was soon struck down by one of the multiple plagues that had been ripping through much of the Mediterranean and Persian world for decades. Another experienced officer by the name of Aurelian inherited the Empire. With military pragmatism in mind, he abandoned the difficult to hold province of Dacia, and attempted to redress the economic crisis by increasing taxes on the wealthy and curbing rampant inflation. Aurelian’s early years as emperor remained focused on defending the central provinces of the Empire, but he was soon able to march on the east to reassert Roman control over the areas subsumed by the Palmyrene Empire in 273. The Persian Empire was troubled by internal issues at this point and was not able to offer any resistance. Almost without stopping, Aurelian turned his army around and by the next year marched into Gaul and reunited the breakaway Gallic Empire under central control, thus earning himself the title ‘Restitutor Orbis’ – ‘Restorer of the World’. Stability was increasingly restored, and soon emperor Probus would rule from 276 to 282 – the longest reign since the Crisis began. His successor, Carus, would be in a strong enough position to launch a massive invasion of the Persian Empire in 283, which indicates the Empire had regained a great deal of its regional hegemony by this point.

It was not just the emperors and the armies that reconstituted the Empire. Although the Empire was fragmented, it was not split apart for an extraordinarily long time, and all three parts retained their Roman cultural identity. Their motivation for independence was not self-governance for its own sake, but it was because of the inability of the central authority to provide for the security of the extremities. External factors also played a part, including the distraction of the Persian Empire with its own issues, and probable fatigue on the part of the European invaders. Although they had found many successes, they had also suffered defeats at the hands of the Romans and presumably had weakened manpower. The rampancy of the plagues also slowed, and local agricultural adjustments were made to a changing regional climate. Economic history is one of those areas that I know just enough about to know how much I do not know. What I can say is that almost every emperor made attempts of varying success at curbing the financial issues plaguing the imperial fisc.

In 284, one of my favorite emperors would come to power. Diocletian is often attributed with saving the Empire from the Crisis, but as just mentioned, a great deal of restoration was completed by previous emperors. What sets Diocletian apart was his reforming vision of the Empire and how he cemented many changes that removed some of the early Empire’s weaknesses. In doing so, the nature of the Roman government was fundamentally modified, and we often call this period the ‘Dominate’ because of the much more obvious authoritarianism of the emperors. In 286, after a few years of sole rule, Diocletian raised an officer, Maximian, as a co-Augustus. This was not new, co-emperors often from the same family had ruled before, but what is important is that it was the first step in developing Diocletian’s new system. In 293, the next step was taken when Diocletian and Maximian raised Galerius and Constantius as co-Caesares, or junior emperors. Thus, in what was a novel system, the Roman Empire was ruled by four unrelated emperors, two senior and two junior. This system is known as the Tetrarchy. Each emperor took a city on an important frontier as their capital, thus spreading their armies and ability to respond to foreign incursion. Whereas emperors in the early period had reigned mostly from Rome, a demilitarized city, now each emperor was protected by his own army, reducing the threat of usurpation. Diocletian also embarked upon extensive reforms in other aspects of the Roman world, including engendering major religious persecutions, enacting a doomed attempt at price controlling, and a reform of the civil and legal administration. To also shore up Rome’s defenses to ward off further foreign invasion, extensive fortifications were constructed along the frontiers, and many of Rome’s cities were protected by new walls.

In perhaps his most stunning move, Diocletian and Maximian became the first Roman emperors to retire. Galerius and Constantius now became the Augusti, and raised two Caesares of their own. Unfortunately, the dynastic principle won out, and Constantius’ son, Constantine, would overthrow the other Tetrarchic emperors and unite the Empire under his sole reign by 324. He did not abandon the Empire’s defenses, however, and the Empire retained much of its power through the fourth century. Although the eventual dissolution of the western Empire in the fifth century is not unconnected to the recovery from the Third Century Crisis, it can be asserted with confidence that the Empire made a full recovery from the catastrophes of the third century.

The Sengoku Period of the Japanese Empire is an example of this. This period began with the Onin War in 1467. Prior to this war, the nation was united under the Ashikaga shogunate. However, the shogun at the time Ashikaga Yoshimasa had no heir so he convinced his brother Ashikaga Yoshimi to leave his life as a Buddhist monk and be proclaimed the heir. But surprise, one year later Yoshimasa had a son Yoshihisa.

Two heirs almost never ends well. Yoshimi was supported by one of the great Japanese families - the Hosokawa clan - and Yoshihisa's mother Hino Tomiko wanted her son to rule so she recruited the Yamana Clan to support Yoshihisa's claim. This is where things get really bad - the Yamana and Hosokawa clans openly fought in the streets of Kyoto. For centuries Kyoto had been above the fray, but blood was shed and the nation wouldn't recover for a long time. The families fought and the nation picked sides, the end result was that there was no evident winner and the unified nation fell apart. The Hosokawas kind of won in the sense that their Ashikaga was in charge of Kyoto, but the fact that they didn't control all of Japan was indicative of a loss.

For the next ~150 years the Daimyo (great names) were mixed on whether they accepted the authority of the Shogun, but the Shogun didn't have the strength to force subservience and the Daimyo fought each other for small territorial gains. The country had no REAL head, the Emperor had no power, the Shogun had no force, everything had fallen apart.

This period ended with a series of three leaders who unified the country. The beginning of the end of the crisis was 1560 CE when Oda Nobunaga "the Great Unifier" won the Battle of Okehazama. In 1560 the Imagawa clan had amassed an army and was marching to Kyoto to end the Ashikaga shogunate, but they had to pass through the lands recently subdued by the Oda clan. The Oda army was greatly outnumbered by the massive nation-crushing force the Imagawa clan had assembled. The Imagawa army didn't bother to ask the Oda clan if they could pass, they decided they could just overrun the territory. They were wrong. Nobunaga was a brilliant general and on a hot Japanese summer day, when the Imagawa army was resting and had removed their armor due to the intense heat and humidity, the Oda forces used a thunderstorm to hide their movements and struck like lightning. They smashed the unprepared Imagawa army in the Battle of Okehazama. Afterwards the name Imagawa dropped in prestige and other Daimyo and samurai pledged fealty to the new strongest lord - Nobunaga.

8 years later, Nobunaga would finish what the Imagawa had started. In 1568, he marched to Kyoto and forced the Yamato Matsunaga clan to surrender the capital to him. In 1573, Nobunaga officially ended the Ashikaga shogunate. The nation was still heavily fractured and Nobunaga would spend the ensuing years up until 1582 crushing other Daimyo and forcing prefectures to submit.

In 1582, Nobunaga was betrayed by his own general Akechi Mitsuhide in what's known as the Honno-Ji incident and Nobunaga was assassinated. A loyal retainer named Toyotomi Hideyoshi became the second of the three leaders when he avenged Nobunaga and killed Mitsuhide. Hideyoshi was now the preeminent leader and the heir apparent to Nobunaga's growing power. In 1590 Hideyoshi accomplished the goal of unifying Japan for the first time since 1467 when he defeated the Hojo clan. All seemed well, until he decided Japan was not enough and launched a series of ill-advised invasions of Korea. The Korean invasions went poorly and Hideyoshi's prestige fell. He died in 1598.

The final of the three leaders and the one who ended the period of war and decline that the Japanese Empire had fallen to was Tokugawa Ieyasu. There's a saying in Japan:

Nobunaga made the pie, Hideyoshi baked it, but Ieyasu got to eat it.

After the death of Hideyoshi, the Toyotomi clan sought to retain their hold on the country but there was a third general. Ostensibly loyal to Oda Nobunaga, but he'd remained sitting back home through the wars of unifications. Waiting. Watching. When Hideyoshi died, Tokugawa Ieyasu stopped waiting.

In 1600 the most important battle in feudal Japanese history took place - the Battle of Sekigahara. Ieyasu's forces had come face to face with the army of Ishida Mitsunari the Toyotomi loyalist. In what is one of the most stunning outcomes of any feudal battle, a significant portion of the Daimyo who had been loyal to Ishida switched sides both shortly before the actual battle and some during the battle which clenched the victory for Ieyasu.

The path was clear for the Tokugawa shogunate to establish itself and thus ended the Sengoku Period and began the Edo Period.

Sources:

Sengoku Jida. Nobunaga, Hideyoshi, Ieyasu: Three Unifiers of Japan by Danny Chaplin (2018)

Japanese Culture: A Short History by Paul H. Varley (1973)

Modern Japan: A Historical Survey by Mikiso Hane (1992)

Samurai Battles: The Long Road to Unification by William De Lange (2020)