As early as the period of Emperor Taizong, the Tang Dynasty had already incorporated the Northeast into the territory, and then the forces in the Northeast began to stir. It was not until Wu Zetian came to power that the forces in the northeast were no longer controlled by the Tang court. Liaodong area was thus out of the control of the Tang Dynasty. Wu Zetian, as the ruler, looked at the frontier tribes ready to move, and had to find a way to solve the problem.
After Wu Zetian reached the summit, she would naturally not sit idly by and turn a blind eye to this small country that should have belonged to the Tang Dynasty but was self-reliant because of the turmoil. She began to plan to attack the Bohai Sea, the real purpose is to get back the lost land, not for the so-called as rumored? Beauty rice? . However, Wu Zetian's attack on Bohai as a rice has been passed down to this day as an anecdote. In fact, it is difficult to distinguish between true and false.
However, after Wu Zetian established the court of Wu Zhou, the original powerful military strength began to decline. At that time, Wu Zetian's power was not stable enough, and the terrain of Bohai Sea was easy to defend but difficult to attack. Therefore, the battle lasted for seven years. Wu Zetian was sure to win the Bohai Kingdom because she didn't want the territory of the imperial court to shrink during her rule, which was not conducive to the development of the imperial court and the consolidation of Wu Zetian's power.