Complex sentence: Koizumi remained in the Diet through the administrations of Abe and Yasuo Fukuda, but announced his retirement from politics on September 25,2008, shortly following the election of Taro Aso as Prime Minister.
Split into simpler sentences: Koizumi remained in the Diet through the administrations of Abe and Yasuo Fukuda. He announced his retirement from politics on September 25,2008, shortly following the election of Taro Aso as Prime Minister.