Complex sentence: On the next day, the Allied force attacked south toward Yenaungyaung and Pin Chaung where the Japanese suffered heavy casualties, but the Allied forces were too weak to hold the oil fields and had to retreat to the north.
Split into simpler sentences: On the next day, the Allied force attacked south toward Yenaungyaung and Pin Chaung. The attack caused the Japanese to suffer heavy casualties, but the Allied forces were too weak to hold the oil fields and had to retreat to the north.