Whenever military conflict arose in any part of the empire, the Imperial Guards would leave the capital to quell the rebellion.
However, Emperor Taizong's northern expedition first sent the elite Imperial Guards to their deaths in one fell swoop, turning them into "Yinlang."
Second, prolonged peace gradually led to the decay of the Imperial Guards.
Even though there hadn't been a large-scale war between Yan and Qian for almost a hundred years, the Imperial Guards of Qian State had deteriorated to the point that they couldn't even quell the empire's internal rebellions.
