"Entering the customs" refers to entering the Central Plains from Shanhaiguan. During the reign of Huang Taiji in the Qing Dynasty, Shanhaiguan was firmly in the hands of the Ming army. Although the Qing army marched into the Central Plains from other passes for looting several times, it finally returned to the pass, so it cannot be said that the Qing army entered the pass. Until 1644, Wu Sangui took the initiative to invite the Qing army led by Dourgen to Shanhaiguan.