The Darkness on Diamondia (1974)

Colonel Morton was sent to Diamondia to report on the war between Earth-descended colonists and the guerilla warriors of the inhuman Irsk. Because something was going terribly wrong - a darkness was setting in, mental confusion was epidemic, and there was evidence of outside interference.
The Darkness was impartial, and Morton's encounters with it were the most disturbing events of his career. For it seemed as if the Outside were deliberately stirring up the planetary pot, mixing minds with minds, and personalities with personalities. But when Morton realized that the only solution might be to find and use the incalculable power of the Lositeen Weapon, he realized also that the decision was too great for any one man - even for all men together - to make. It is loosely based on the short story "Humans, Go Home!" from 1969.