"The difficulties of conducting espionage against the Soviet Union in the Soviet Union were such that historically the Agency had backed away from the task"

This quote by Aldrich Ames is referring to the problems of conducting espionage against the Soviet Union while inside the Soviet Union. Historically, the Agency (most likely the CIA) had prevented this task because of the severe trouble of doing so. The Soviet Union was a shut society, and it was difficult to gain access to the info the Agency wanted. In addition, the Soviet Union was very suspicious of outsiders, and it was hard to move around the nation without exciting uncertainty. The Agency was additionally aware of the Soviet Union's comprehensive counterintelligence capacities, that made it challenging to run without being spotted. Every one of these variables combined to make espionage against the Soviet Union in the Soviet Union a tough and unsafe job, and one that the Agency had historically stayed clear of.