 Complete the following phrase:  The Song Empire was prosperous during Zhao Kuangyi Emperor Taizongs reign despite the threats posed by the northern nomadic empires, such as the Khitan-led Liao Empire, Jurchen-led Jin Empire and Tangut-led Western Xia. In 1115, the Song aligned themselves with the Jin regime in the hope of capturing the Sixteen Prefectures. However, this alliance allowed the Jurchens to learn of the Songs military weaknesses, namely a lack of capable commanders and armies primarily composed of mercenaries and poorly trained convicts. After the fall of the Liao empire, the 