1660s, from un- (1) "not" + conditional (adj.). Related: Unconditionally. Unconditional surrender in the military sense is attested from 1730; in U.S., often associated with Civil War Gen. Ulysses S. Grant and the taking of Fort Donelson.
The ringing phrase of Grant's latest despatch circulated through the North like some coinage fresh from the mint, and "Unconditional Surrender," which suited the initials of his modest signature, became like a baptismal name. [James Schouler, "History of the United States of America," Dodd, Mead & Co., 1899].