Both sides feared their opponent might obtain first strike, but did not want it themselves.

Problem is that some actions that help prevent your opponent from obtaining first strike move you closer to first strike.

For example, having nuclear missiles in submarines makes it very hard for your opponent to take out most of your nuclear weapons in a first strike, but it also gives you the capability to fire from closer range, giving the opponent less time to launch their missiles before your missiles take them out.

This would be similar, as there would be less time between launch and impact.