Complex sentence: Mil proposed the design to the heads of the Soviet armed forces, and while he had the support of a number of strategists, he was opposed by several more senior members of the armed forces who believed that conventional weapons were a better use of resources.
Split into simpler sentences: Mil proposed the design to the heads of the Soviet armed forces. While he had the support of a number of strategists, he was opposed by several more senior members of the armed forces, who believed that conventional weapons were a better use of resources.