He was balanced on a knife-edge between life and death.

他处关头。

A life-or-death situation can reveal whether somebody is panicky or coolheaded, whether the person is selfish or thinks about others. This also is true for a nation-state.

关头的表现展示一个人是惊惶还是沉着，自私还是有爱心，同，对一个国野也是一样。