Complex sentence: Fukuoka was sometimes called as the Port of Dazaifu (大宰府, 15km southeast from Fukuoka), which was an administrative capital in 663 A.D., but a historian proposed that a prehistoric capital was in the area.
Split into simpler sentences: Fukuoka was sometimes called as the Port of Dazaifu (大宰府, 15km southeast from Fukuoka). Dazaifu was an administrative capital in 663 A.D., but a historian proposed that a prehistoric capital was in the area.