Complex sentence: Thus, although Midway was the Allies'first major victory against the Japanese, it did not radically change the course of the war; instead it was the cumulative effects of the battles of Coral Sea and Midway that reduced Japan's ability to undertake major offensives.
Split into simpler sentences: Thus, although Midway was the Allies'first major victory against the Japanese, it did not radically change the course of the war. Rather, it was the cumulative effects of the battles of Coral Sea and Midway that reduced Japan's ability to undertake major offensives.