A Matter of Logistics

They overpopulated and polluted their home world before their sputtering on-again off-again space program finally took hold. They explored their local system and then developed near-light-speed drives, which opened relatively near worlds they could settle straight out or engineer into livable ecologies. The "relative" was the difficulty in a relativistic universe. Deep-space crews lived by a different clock and outlived planet-bound dynasties, becoming an advantaged culture that...
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They overpopulated and polluted their home world before their sputtering on-again off-again space program finally took hold. They explored their local system and then developed near-light-speed drives, which opened relatively near worlds they could settle straight out or engineer into livable ecologies. The "relative" was the difficulty in a relativistic universe. Deep-space crews lived by a different clock and outlived planet-bound dynasties, becoming an advantaged culture that asserted, as by right, rule over their home world and its colonies. Eventually the planet-bound rebelled and won a vicious war when home-world scientists solved faster-than-light travel before the space-borne. Surviving fleets of the space-borne were banished into deep space in their plodding vessels and essentially forgotten for a hundred years until their new FTL war fleets appeared above colony worlds. The exiles had found homes aplenty in deep space, and made first contact with other intelligent species, none of which had achieved significant space travel. The exiles assigned themselves the role of benevolent gods and raised a mighty force of aliens that pressed close to the home world before being beaten off. Stalemate ensued. The exiles were confronted with logistical issues: how to encircle the enormous globe of space occupied by the home world and all its colonies. As a matter of logistics it was simpler to build arsenals on varying distant worlds than transport materiel those titanic distances; and simpler to recruit millions of sentient dupes as cannon fodder than risk their own necks. Home-world intelligence traced the enemy to a hitherto-unknown world full of satisfyingly violent beings who stood at the dawn of their own space age. So they recruited a penetration agent with peculiarly strong mental powers, equipped him with a perfectly cloned body, and sent him to that isolated island in space...known to its inhabitants as Earth.