《北史·列传·卷九》
两人一向有深仇，这一来势力均衡，谁也不能首先发难，可说是防御边患的上策啊！
两人素有深仇，其势莫能先发，此御边之上策。
The two have always had a deep hatred, and now that their strength is balanced, neither of them can attack first. It can be said that it is the best strategy to defend against border troubles!