Complex sentence: During the planning for the attack on Pearl Harbor, Admiral Yamamoto insisted on using carrier planes to execute the assault and was only ensured the approval of his plan through threatening to resign if the navy decided on another course of action (Thomas 2006).
Split into simpler sentences: During the planning for the attack on Pearl Harbor, Admiral Yamamoto insisted on using carrier planes to execute the assault. However, he only ensured the approval of his plan through threatening to resign if the navy decided on another course of action (Thomas 2006).