Complex sentence: Admiral Chester Nimitz, Commander in Chief, U.S. Pacific Fleet, determined to take the opportunity to engage them: losing Midway would have been a very serious threat because the Japanese then could easily take Hawaii and threaten the West Coast of the United States.
Split into simpler sentences: Admiral Chester Nimitz, Commander in Chief, U.S. Pacific Fleet, determined to take the opportunity to engage them. Losing Midway would have been a very serious threat because the Japanese then could easily take Hawaii and threaten the West Coast of the United States.