This struggle is sharp and complex. We can never let down guard.

这场斗争是尖锐复杂的，我们决不能掉以轻心。

《新英汉大辞典》

You can't let your guard down.

你不能掉以轻心。

《柯林斯英汉双解大词典》

Let their guard down.

让他们放松警惕。

youdao

But,he adds, "I do not think it is time to let our guard down.

VOA: standard.2009.05.03

When you are running well in anything, the natural tendency is to overestimate your ability, let down your guard and lose your perception of risk.

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Nor can we let down our guard against a worldwide threat.

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Manning won't ever let down his guard enough to show doubt.

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