They were caught flat-footed by the attack.

他们冷不防遭到了攻击。

《牛津词典》

The unrest is a blow to the police, caught flat-footed and unable to ensure order.

这场动乱狠狠地打击了警方，暴露出他们对突发事件毫无准备、对维持秩序无能为力的一面。

youdao

In order to "avenge" I rushed into the "enemy" of the "barracks" the enemy caught flat-footed.

为了“报仇”我冲入“敌人”的“军营”将敌人打了个措手不及。

youdao

It is, in such a circumstance, easy to get caught flat-footed by pursuing yesterday's strategy today.

CNN: Going to the People

Lawmakers, lobbyists, and even mainstream beltway advocacy groups fighting the bills were caught flat-footed, and retreated.

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It was caught flat-footed when young athletes stopped playing multiple sports, which had required lots of different shoes and apparel.

FORBES: Magazine Article