Formation of the CoDominium
The point of departure of Jerry Pournelle's history is the establishment of the CoDominium (CD), a political alliance and union between the United States of America and a revitalized USSR. This union, achieved in the name of planetary stability, reigns over the Earth for over a hundred years. In that time, it achieves peace of a sort, as well as interstellar colonization, but at the price of a complete halt in both scientific and political evolution.
Corruption and social decay force the CoDominium's BuReloc (Bureau of Relocation) to forcibly transport people from Earth to offworld colonies. This mass expulsion is made possible by the Alderson Drive, a device that allows instantaneous travel across distances of light-years. The starlanes are patrolled by the CoDominium Armed Forces, an elite fighting force created from the French Foreign Legion. The Navy, in particular, recognizes what the politicians and common people of Earth do not: that Earth is headed for disaster, and their primary mission is to remove as many people from Earth as possible before the holocaust.
In due time, the CoDominium crumbles under mounting nationalism worldwide. The governments of both the United States and Soviet Union regain their thirst for world supremacy, while the lesser nations chafe from years of CD oppression. The CD begins to lose support and is forced to reduce the size of the Fleet again and again, despite the best efforts of loyalists like the Blaine family and Grand Admiral Lermontov. As a result, several underdeveloped colony worlds are given their independence, prepared or not. Civilization collapses on some of these newly freed planets and results in mass deaths.
Disbanded CD Marine units band together into mercenary organizations (for example Falkenberg's Mercenary Legion) and hire themselves out to fight in the former colonies, much as the earlier 'condottieri' of Renaissance Italy. Later, major worlds begin hiring out their national armies, and mercenary work becomes an industry. Eventually, mercenaries become legal in warfare, thanks to the "Laws of War", and are a major factor in colony warfare.
On Earth, the inescapable result is the Great Patriotic Wars, the long-delayed Third World War that begins and ends with massive nuclear exchanges. Much of Earth is devastated; civilization collapses and much of the surface is rendered temporarily uninhabitable.
Empire of Man
Before the end, the bulk of the CoDominium Fleet evacuates their families from Earth and resettle them on Sparta (for the Americans and Europeans) and St. Ekaterina? (for the Russians and Asians). Within a few years, the Fleet swears allegiance to King Lysander I of Sparta. Sparta and the Fleet soon begin the Formation Wars, taking advantage of the political, economic, and technological vacuum caused by the collapse of Earth to reunify the human colonies under Sparta's leadership and form the Empire of Man?.
For several hundred years, the Empire is the sole government of humanity. Among its many worlds is Sauron, where the culture has grown militaristic and adheres to a literal interpretation of the philosophy of Nietzsche, namely that "man is something to be surpassed." In service of this aim, they engage in extensive genetic modification and eugenic breeding programs to turn themselves into supersoldiers known in the galaxy at large as the Sauron Supermen. Bristling under Imperial hegemony, in the 27th century they lead several worlds into open revolt.
The Secession Wars are long and fierce, spanning decades. The Empire has a big edge in numbers, but it is almost matched by Sauron's military superiority. It is the Saurons' arrogance that leads to their downfall. For them, the war is to be waged free of any constraints or mercy and is run with the cold rationality of professionals. For the subjects of the Empire, it becomes a war of extermination, a life-or-death struggle for survival, and their passion is not accounted for in Sauron calculations. The war ends with Sauron being completely destroyed.
The Empire's victory is Pyrrhic though, as the Imperial forces have mostly been sacrificed to crush the Saurons. With the Empire weakened, new factions spring up, including claimants to the Imperial throne. It is reduced to a core of loyal planets, and marginal worlds are isolated. Many, their industries lost to orbital bombardment, descend into barbarism.
Second Empire of Man
In the thirtieth century, Lysander IV of Sparta proclaims the formation of the Second Empire of Man, and pledges to unite humanity by force if necessary, in order to prevent future wars of the magnitude of the one that ended the First Empire. Many worlds quickly accept; others, known as outies, continue to resist the Empire's hegemony. Against this backdrop of renewal and conflict, humanity makes its first contact with another intelligent, spacefaring species: the Moties, who represent a far more grave threat than even the Sauron Supermen. The contact between the two species would have a dramatic impact on both civilizations.