Complex sentence: At the beginning of the Warring States Period, Zhao was one of the weakest states but gained strength during the reign of King Wuling of Zhao: by the end of the period, Zhao was the only state strong enough to oppose the mighty Qin.
Split into simpler sentences: At the beginning of the Warring States Period, Zhao was one of the weakest states but gained strength during the reign of King Wuling of Zhao. By the end of the period, Zhao was the only state strong enough to oppose the mighty Qin.