《辽史·列传·卷六》
大军回，留延寿攻战掠地于贝、冀、深三州。
师还，留延寿徇贝、冀、深三州。
The main army returned, and Yanshou was left behind to attack and occupy land in the three prefectures of Bei, Ji and Shen.
<s>[INST] Identify the key themes in this Classical Chinese text and explain them in English: 师还，留延寿徇贝、冀、深三州。 [/INST] The main army returned, and Yanshou was left behind to attack and occupy land in the three prefectures of Bei, Ji and Shen.</s>
Identify the key themes in this Classical Chinese text and explain them in English: 师还，留延寿徇贝、冀、深三州。
user
The main army returned, and Yanshou was left behind to attack and occupy land in the three prefectures of Bei, Ji and Shen.
assistant