Complex sentence: During the war, it is estimated that approximately 5% of the national population were involved in some form of resistance activity, while some estimates put the number of resistance members killed at over 19,000; roughly 25% of its "active" members.
Split into simpler sentences: During the war, it is estimated that approximately 5% of the national population were involved in resistance activity. Some estimates put the number of resistance members killed during the war at over 19,000, roughly 25% of its "active" members.