The beauty song - 佳人曲

The movie 'House of Flying Daggers (<span lang="zh">十面埋伏</span>)' contains a song sung by Zhang Ziyi (<span lang="zh">章子怡</span>) called 'The Beauty Song (<span lang="zh">佳人曲</span>)':

北方有佳人,绝世而独立。
一顾倾人城,在顾倾人国。
宁不知倾城与倾国。
佳人难再得。

Which translates to something like the following:

In the north there is a beauty: Surpassing the world, she stands alone.
A glance from her will overthrow a city; A second glance will overthrow the State.
Don't I know she can overthrow the city and the State?
But such a beauty cannot be found again!

This is a poem by the Han Dynasty poet Li Yannian (<span lang="zh">李延年</span>).