Complex sentence: The Douglas DB-7s, meanwhile, were to have gone by freighter to Africa, where they would be assembled and ferried to China, but the attack on Pearl Harbor caused the program to be aborted.
Split into simpler sentences: The Douglas DB-7s, meanwhile, were to have gone by freighter to Africa, where they would be assembled and ferried to China. However, the attack on Pearl Harbor caused the program to be aborted.