《北史·列传·卷四十二》
开初，文宣帝在位时，北周的军队常害怕北齐的军队西渡黄河，常在冬天凿开河冰，以作防守。
初，文宣时，周人常惧齐兵之西度，恆以冬月，守河椎冰。
In the beginning, during the reign of Emperor Wenxuan, the Northern Zhou army always feared that the Northern Qi army would cross the Yellow River heading west, and would often break the river ice open in the winter to mount their defense.
<s>[INST] Identify the key themes in this Classical Chinese text and explain them in English: 初，文宣时，周人常惧齐兵之西度，恆以冬月，守河椎冰。 [/INST] In the beginning, during the reign of Emperor Wenxuan, the Northern Zhou army always feared that the Northern Qi army would cross the Yellow River heading west, and would often break the river ice open in the winter to mount their defense.</s>
Identify the key themes in this Classical Chinese text and explain them in English: 初，文宣时，周人常惧齐兵之西度，恆以冬月，守河椎冰。
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In the beginning, during the reign of Emperor Wenxuan, the Northern Zhou army always feared that the Northern Qi army would cross the Yellow River heading west, and would often break the river ice open in the winter to mount their defense.
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