Complex sentence: Li Shimin's forces entered the palace and, under the intimidation of Li Shimin's forces, Emperor Gaozu agreed to create Li Shimin crown prince, and two months later passed the throne to him (as Emperor Taizong).
Split into simpler sentences: Li Shimin's forces entered the palace and, under the intimidation of Li Shimin's forces, Emperor Gaozu agreed to create Li Shimin crown prince. Two months later, Emperor Gaozu passed the throne to Li Shimin (as Emperor Taizong).