Complex sentence: Midway; and later from Guam, played a major role in defeating Japan, even though submarines made up a small proportion of the Allied navies--less than two percent in the case of the US Navy.
Split into simpler sentences: Midway; and later Guam, played a major role in defeating Japan. This was the case even though submarines made up a small proportion of the Allied navies--less than two percent in the case of the US Navy.