The tides were turning at this point in the Second World War. The Allied Forces were pushing back Germany, and the USA started its drive to take over the Pacific islands now controlled by the Japanese. Slowly but steadily, they worked their way westwards towards mainland Japan. By gaining control of the islands near Japan, including Iwo Jima and Okinawa, it would provide a suitable launchpad, of sorts, to start their attack on Japan.
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