>>509093007The Romans had so many civil wars it could be considered a part of their culture.
>America had very little power anywhere until almost the 80s when we talk about world power.First of all, this isn't true. The US was already the greatest economic powerhouse on the planet by the early 20th century. And after WWII we were one of two global powers to which the rest of the world had to kneel. But also, we're not talking about global power.
Very few empires have ever had substantial global power. It pretty much only started to be a thing with European colonial powers. And, before the US, the British were the only ones to effectively take over the world. That is not what makes you an empire.
There isn't any one super solid definition. But being the master of an entire hemisphere definitely counts. And that's what the US was after we humiliated the Mexicans and proved that no one else in the Americas could hold a candle to us. And even people at the time understood that. One of the primary objections in the US to the Mexican American War was that it was an act of imperialism.