《汉书·传·匈奴传下》
边境上已经十分空虚，不能供应军粮，在内地各郡县封国征集，又供应不上去，这是第二个难处。
边既空虚，不能奉军粮，内调郡国，不相及属，此二难也。
The border is too desolate and cannot support military supplies. It will not be able to go to the counties and prefectures in the interior to collect supplies, and that is the second difficulty.