Complex sentence: Nevertheless, in September 1983, the CIA assigned Ames back to the SE division in Washington, placing him "in the most sensitive element" of the Department of Operations, that was responsible for Soviet counterintelligence.
Split into simpler sentences: Nevertheless, in September 1983, the CIA assigned Ames back to the SE division in Washington. His reassignment placed him "in the most sensitive element" of the Department of Operations, which was responsible for Soviet counterintelligence.