Complex sentence: When the wars between the Taira and Minamoto began it was Ogata no Saburo Koreyoshi and his armies that played the key role in securing Minamoto no Yorimitsu's control of Japan.
Split into simpler sentences: When the wars between the Taira and Minamoto began. It was Ogata no Saburo Koreyoshi and his armies that played the key role in Minamoto no Yorimitsu's securing control of Japan.