Complex sentence: The system was being developed to replace the previous Talos, Terrier, and Tartar missile systems in service in the early 1960s; the earlier systems were determined to be vulnerable to saturation by large scale missile attacks.
Split into simpler sentences: The system was being developed to replace the previous Talos, Terrier, and Tartar missile systems in service in the early 1960s. The earlier systems were determined to be vulnerable to saturation by large scale missile attacks.