Complex sentence: The majority of Mid-Canada Line stations were used only briefly from the late 1950s to the mid- 1960s, as the attack threat changed from bombers to ICBMs; the early warning role passed almost entirely to the more capable DEW Line further north.
Split into simpler sentences: The majority of Mid-Canada Line stations were used only briefly from the late 1950s to the mid- 1960s, as the attack threat changed from bombers to ICBMs. As the MCL was closed down, the early warning role passed almost entirely to the more capable DEW Line further north.