They were constantly on the alert not to be taken by surprise.

他们一直处于常备不懈状态以防突然袭击。

《新英汉大辞典》

Spoiler alert: If you haven't finished the video, I'm now going to ruin the surprise.

告密者警告：如果你还没看完视频的话，下面我可要扫你的兴了。

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We were constantly on the alert against any surprise attack.

我们时时刻刻都在提防突然袭击。

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