《汉书·传·匈奴传下》
匈奴那裹秋冬季十分寒冷，春夏季常刮大风，衹有多带大锅木炭才能应付，然而又嫌太重，不可担负。吃干粮、喝白水，从而度过一年四季，军队有发生瘟疫的危险，所以前代攻打匈奴，不过一百天，不是不想延长时间，是情况不允许，力量达不到，这是第四方面的难处。
胡地秋冬甚寒，春夏甚风，多赍釜鍑薪炭，重不可胜，食糒饮水，以历四时，师有疾疫之忧，是故前世伐胡，不过百日，非不欲久，势力不能，此四难也。
The Xiongnu is very cold in the fall and winter and has strong winds in the spring and summer, so only large pots and charcoal can be brought to cope, but they are too heavy to carry. Eating dry food and drinking boiled water to endure the changes of the seasons, there is a risk of the army developing an epidemic, and therefore in previous generations, when attacking the Xiongnu, only 100 days at most were spent, not because they did not want to stay longer, but because the situation did not allow it and the strength could not be sustained; this is the fourth difficulty.