Complex sentence: Roosevelt's order came three days after Hitler broke the non-aggression pact and invaded the Soviet Union, at which time the Communist Party quickly directed its members to get behind the draft and forbade participation in strikes for the duration of the war (angering some leftists).
Split into simpler sentences: Roosevelt's order came three days after Hitler broke the non-aggression pact and invaded the Soviet Union. The Communist Party now immediately directed its members to get behind the draft, and it also forbade participation in strikes for the duration of the war (angering some leftists).