《资治通鉴·晋纪·晋纪三十七》
北魏长孙嵩征讨柔然，到了大漠以北才回来，柔然军在牛川追上长孙嵩部队并把他们包围。
魏长孙嵩至漠北而还，柔然追围之于牛川。
Northern Wei Changsun of Song led a military expedition to attack Rouran, and turned back only after reaching the area north of the vast desert. The Rouran army caught up with Changsun of Song’s troops at Niu’chuan and surrounded them.
<s>[INST] Identify the key themes in this Classical Chinese text and explain them in English: 魏长孙嵩至漠北而还，柔然追围之于牛川。 [/INST] Northern Wei Changsun of Song led a military expedition to attack Rouran, and turned back only after reaching the area north of the vast desert. The Rouran army caught up with Changsun of Song’s troops at Niu’chuan and surrounded them.</s>
Identify the key themes in this Classical Chinese text and explain them in English: 魏长孙嵩至漠北而还，柔然追围之于牛川。
user
Northern Wei Changsun of Song led a military expedition to attack Rouran, and turned back only after reaching the area north of the vast desert. The Rouran army caught up with Changsun of Song’s troops at Niu’chuan and surrounded them.
assistant