Hunter Patrol

"Hunter Patrol" is a 1959 sci-fi story co-authored by H. Beam Piper and John J. McGuire. A soldier travels into the future to kill a dictator. After completing the mission and returning home to his present time, he doesn't remember what he has done. With him he has an envelope with the formula for a fizzy drink that would make him able to conquer the whole world... possibly.