Complex sentence: Among his achievements was proving in war games in 1932 that Pearl Harbor was vulnerable to a naval aerial attack, only to have his findings dismissed by his superiors until the Imperial Japanese Navy attacked the base in 1941 in the same manner that Yarnell predicted.
Split into simpler sentences: Among his achievements was proving, in 1932 war games, that Pearl Harbor was vulnerable to a naval aerial attack. His findings were dismissed by his superiors until the Imperial Japanese Navy's Pearl Harbor attack was just as Yarnell predicted.