McNamara later concluded that counterforce was not likely to control escalation but to provoke retaliation. It is accurate enough to be a first strike, counterforce, or second strike weapon. An ideal counterforce attack would kill no civilians. A counterforce exchange was one scenario mooted for a possible limited nuclear war. Counterforce orthosis has a circumferential structure surrounding the arm. In nuclear warfare, enemy targets are divided into two types&colon; counterforce and countervalue. Counterforce is available on Earth from gravity. The necessary counterforce is provided by the thumb. During the Reagan administration, there was a return to a strong counterforce strategy through NSDD-13. "Part 4&colon; The Counterforce" is made up of 12 episodes.