Complex sentence: By the early-to-mid 1970s Brezhnev had established a strong enough power base to effectively become supreme leader, however, Kosygin was to retain his position until 1980 when he resigned due to bad health.
Split into simpler sentences: By the early to mid 1970s Brezhnev had established a strong enough power base to effectively become supreme leader. However, Kosygin was to retain his position until 1980, when he resigned due to bad health.