Li Xuan's northern campaign route, after passing through the border, closely resembled the route taken by Emperor Taizong on his second northern expedition.
All seven hundred thousand troops passed through Zhangjiakou to Dalu city and then crossed the Hanhai, also known as the great desert or the Gobi Desert of later times, using two clear water sources to the north and south and Saihan Mountain, arriving in MoBei. Subsequently, they directly attacked Ulan Bator from Qihu Mountain and Jiesheng Ridge.
The so-called 'Hanhai' or great desert is later known as the Gobi Desert, spanning thousands of miles, extending west to the Tian Mountain and east to the Great Xingan Mountain. Meng Wu used this as the border, dividing the area into MoNan and MoBei.
The MoNan area, where Li Xuan had defeated Tuotuobuhua last April, decimating tens of thousands of Datan, had already become indefensible to the Oirat People.
