罗马帝国的灭亡是历史上的一个关键时刻。

The ___fall of the Roman Empire was a pivotal moment in history.

在旧系统垮台后，社会经历了快速变革。

After the ___fall of the previous system, society underwent rapid transformation.

专制政权的意外垮台引发了广泛的庆祝。

The unexpected ___fall of the authoritarian regime sparked widespread celebration.