Complex sentence: This is not say that the era of Emperor Haile Sellasie, following the devastating Italian occupation (1935-1941), did not see significant advancements in many areas, including in new frontiers such as manufacturing and transportation (e.g. aviation).
Split into simpler sentences: This is not say that the era of Emperor Haile Sellasie, especially in the decades following the devastating Italian occupation (1935-1941), did not result in further development. Significant advancements were made in many areas, including new frontiers such as manufacturing and transportation (e.g. aviation).