Complex sentence: At the beginning of the war twenty-five missions was considered a full tour of duty, but once pilots became more efficient and effect in the air, it was then raised to thirty-five missions before the end of the war.
Split into simpler sentences: At the beginning of the war twenty-five missions was considered a full tour of duty. Once pilots became more efficient and effect in the air, they then raised it to thirty-five missions, before the end of the war.