Complex sentence: After World War I, opposition to the scheme gave rise to a movement to re-segregate the executive and engineering branches because during the war, officers were not able to be equally proficient at both engineering and executive duties.
Split into simpler sentences: After World War I, opposition to the scheme gave rise to a movement to re-segregate the executive and engineering branches. During the war, officers were not able to be equally proficient at both engineering and executive duties.