In other words, PD59 explored a "warfighting" doctrine that suggested that nuclear plans might change during a war, and that nuclear weapons were to be used in combination with conventional weapons. Carter's Secretary of Defense, Harold Brown, emphasized selective counterforce, but also explicitly threatened the Soviet leadership themselves. Major improvements in U.S. command, control, communications and intelligence (C3I), including making elements survivable during a nuclear war, were instituted to make the PD-59 doctrine feasible. By 1982, SIOP-5 contained more than 40,000 possible targets in four categories: