The Anguished Dawn

Earth has been devastated after a near miss by a white-hot protoplanet ejected from Jupiter. The poles and climatic bands have been shifted, continents and oceans resculpted in titanic cataclysms, and civilization as we know it has ended.
Only the colony of Kronia, established among Saturn's moons, preserves technology and human culture. While the few bands of dazed survivors left on Earth revert rapidly to brutality and barbarism, the Kronian culture is already opening up new realms of physics that will carry humankind to the stars.
The Kronians have a vision of founding a new civilization on the reborn Earth, building upon foundations of freedom and the power of unbridled human creativity worthy of a stargoing civilization, and avoiding the destructive influences of conquest and exploitation. So when conditions on Earth stabilize sufficiently, Landen Keene, previously a nuclear propulsion engineer in the world that no longer exists, returns with the first Kronian reconnaissance mission to establish a base in what was a region of Africa, and commence the task of beginning anew.
But not everyone wants a free society in which individual worth is measured by ability and contribution, not possessions and power. The Terrans who were brought to Kronia included people whose status and recognition on Earth stemmed from wielding power of the kind that the old order understood. When their bid for a bigger part in running things fails, they resort to deception, violence, and the methods that previously served them well.
They plan a series of coordinated moves to seize the newly established base on Earth, and initiate a new Terran order built on the old principles of force, subjugation, and domination all over again. Isolated by distance from the hope of any immediate help from Saturn, and with only a few helpers and pitifully slim resources at his disposal, Keene emerges as the only hope of improvising an effective resistance.
But maybe that was precisely why he was sent there.