A tiger caught a fox in a forest, and was just about to eat it, when the fox
said, 'You mustn't eat me. I was sent by Heaven to rule the animals. By eating
me, you will violate the command of Heaven. If you don't believe me, just follow
me to see whether the animals are of me.' The tiger agreed, and followed the fox
as it walked around the forest. The animals all ran away on seeing them. The
tiger thought they were of the fox, so he let it go. He didn't realise that it
was him that the beasts were really of.

This idiom means relying on another's power to bully or frighten others.

老虎在山林里捉到了一只狐狸，要吃掉它。狐狸连忙说：“你不能吃我，我是天帝派来统治百兽的。你要吃了我，就违抗了天帝的命令。你不信，就跟我到山林里去
一趟，看百兽见了我是不是都很害怕。”老虎相信了狐狸的话，就跟在狐狸的后面走进山林。百兽见了果然都纷纷逃命。老虎以为百兽真的害怕狐狸而不知道是害怕
自己，于是就把狐狸给放了。

“狐假虎威”这个成语用来比喻倚仗别人的势力去欺压人或吓唬人。

狐假虎威 【翻译】Basking in Reflected Glory.