Complex sentence: During the late 1970s, the Soviet Union fielded a large number of heavy, increasingly accurate, MIRV ICBMs (like the SS-18, defined by throw weight) which seriously threatened the survival of Minuteman III missiles in their silos.
Split into simpler sentences: During the late 1970s, the Soviet Union fielded a large number of heavy (defined by throw weight), increasingly accurate MIRV ICBMs like the SS-18. These had the accuracy and numbers needed to be a serious threat to the survival of Minuteman III missiles in their silos.