The Ivanhoe Gambit (Time Wars, 1)

FIRST ACE ORIGINAL EDITION. Jan 1984 mass market paperback, Simon Hawke (The Wizard of 4th Street). In the 27th Century, international disputes are resolved through time travel, "clocking" soldiers from the future into conflicts of the past. It seemed like an elegant idea: wartime industries drove the economy without actually putting nations through the physical ravages of war. And since the past already happened, history could not be changed. So went the theory. What could possibly go wrong?