Its pointless to compare ancient empires to today. When "Rome ruled the world" 98% of the land "they ruled" was empty and worthless with little enclaves of people here and there who were so isolated they didn't even know they were "being ruled"! But how long have empires lasted in modern times with modern transportation, communications and weaponry? How long did Germany's effort to take over Europe last?
I just can't see the idea of China coming half way around the world to invade the US. It doesn't make sense in any way. Furthermore I'm not suggesting we bury our weapons and become defenseless lambs. We still have an enormous nuclear deterrent and every country knows if they tried to invade us their homeland would be turned into a radioactive pile of rumble.
The threat the US faces today is not an external enemy Bob- it's internal revolt and chaos from it's own citizens who become fed up with the inability to find good jobs and decent wages- the ever-growing disparity between the 'haves' and the 'have-nots' struggling to pay bills month to month while the rich just keep getting richer.
Right now there are people in the US stockpiling food and weapons and preparing for the next war- but it won't be against China- it will be against our own government and the ruling elite. The only way to prevent this is for us to stop squandering our resources overseas and start addressing the problems right here at home!