The focus of the explanation is the transition from a hunter-gatherer economy to an agrarian one. Uncivilized peoples near the borders of civilization learn advanced elements of agriculture from it, their population explodes, and they overrun their civilized neighbors. There are variations in the theme, but essentially that is the idea.
Although rudimentary farming of a sort was quite widespread before these dramas began, it was not everywhere as well settled, well managed or as efficient as in the centers of civilization, and wild game was still a larger part of the diet for the outsiders. Sumeria, Assyria, Persia, Greece and Rome were players in approximately the same theater, each in turn the rulers of the "world," later to become civilized and in turn be overrun by the fringe.
Often in debates supporters of homosexuality will note that it was acceptable in such "great civilizations" as ancient Greece and Rome. As can be seen here though, rather than great civilizations, the Greece and Rome that accepted homosexuality were "newbie" civilizations, an odd mix of culture and barbarity.
The island security of civilized England was invaded three times within this scope; by Rome, by the Vikings, then later by the Normans. A good question is whether the Norman conquest should count as fierce hunter-gatherers over farmers. The French words introduced into English through that conquest are often counted as the more civilized. "Perspire" is considered more "genteel" than "sweat" for example. The Normans might be counted though as having been more vunerable to invasion than island England for so many years as to adapt more warlike, unsettled attitudes. Later the "sun never set" on the British Empire, but that was as noted later.
Can the fierce hunter-gatherer over the settled farmers explain social or political upheaval since then though? No it cannot, not without extreme modification. The ancient civilizations in the new world do not seem to fit the paradigm well. It may be that they did but there are no records. There might have been such conquests in Sumeria before records also. When Europeans arrived in the New World the civilizations they found are believed to be at a stage of development similar to that of very ancient Sumeria. In the clash between the old and new worlds the old had a 6,000 year advantage.
After Old World settlement of America the clash between the city and the country undergoes major modification. The Federalists were for a strong central government and laws, the Anti-federalists were against central government and more for freedoms. Curiously perhaps, the Federalists later became Republicans and the Anti-federalists became Democrats. That is an inversion and is explained in greater detail in the article on Cultural Inversion.
First it is important to get a firm grasp on the basic theme. It is in a very basic form here to keep the focus on the key ideas. Many writers known for their work on cycles in history deal with more or less similar ideas and considerably more detail. Hegel's Thesis-Antithesis-Synthesis is an example.
Cicero
There are numerous writers on such cycles as capitalism-socialism-withering. None of the systems are good predicters of what will follow. They are more useful for a somewhat better understanding of what has already happened.
To explain events after the Roman Empire fell using the civilization-fringe expansion-overthrow paradigm there has not been "one known world government" since Rome's yet. There are increasingly stronger signs of one forming, albeit as a more or less organized collection of governments with considerable cultural diversity, still far more than Rome's Empire, rather than a single government.