罗马帝国曾横跨三大洲。

The Roman ___empire once spanned across three continents.

历史学家们经常争论一个庞大帝国衰落的精确原因。

Historians often debate the precise causes behind the decline of a vast ___empire.

通过一系列军事征服，他试图建立一个持久的帝国。

Through a series of military conquests, he sought to build an enduring ___empire.