Complex sentence: During the Cold War Murmansk was a center of Soviet submarine and icebreaker activity and, after the dissolution of the Soviet Union, the nearby city and naval base of Severomorsk remains the headquarters of the Russian Northern Fleet.
Split into simpler sentences: During the Cold War Murmansk was a center of Soviet submarine and icebreaker activity. After the dissolution of the Soviet Union, the nearby city and naval base of Severomorsk remains the headquarters of the Russian Northern Fleet.