The events of the last few years since 9/11 have often felt as though the United States under George W. Bush was repeating the pattern of the Roman Empire. What follows is a bit of random thought - a "comparison by analogy" of events between Rome and modern history. Since individual events differ considerably in their details, I will focus more on the effect of those events on either Rome, or the United States as appropriate.
The raw timeline looks like this:
First Punic War
Third Punic War
Roman Asia Province
Gothic Invasions of Rome
Where the Third Punic War established Rome as a military power in the Mediterranean - and one that could not be ignored - the Second World War pulled the United States onto the world stage as a lead player. The destruction of Nagasaki and Hiroshima showed the Americans to be capable of ruthless violence when provoked. Just as Cato insisted that Rome completely destroy Carthage, Washington destroyed two Japanese cities before accepting surrender.
Later, both the Korean and Vietnam conflicts showed that the US forces were not invincible. Similarly, although Rome claimed victory over the pirates that were marauding shipping from time to time, the Pirates kept on reappearing later - showing themselves to be an enemy that Rome could not fully quash. (There are also many analogies that compare the Roman-era pirates with modern day terrorist operations - but that's another topic)
Dig still further, and we hit Rome's involvement in Asia Province. Rome's "Asia Province" was never really "conquered", although Rome routinely sent troops and a governer to the region to harvest taxes. In 1979, the Iranian revolution that overthrew the Shah, and resulted in US embassy staff being taken hostage for a year or so gave the US a pretty good black eye on the world stage. Rome's shakey control over Asia province and the US's loss of influence over Iran's affairs are one more clue in the puzzle. One that demonstrates that for all of the US army's muscle, they cannot control those that do not wish to be controlled.
Iraq and Afghanistan underline a problem. Just as occupying Gaul ultimately proved to be more than Rome could afford to track, Iraq and Afghanistan are proving very difficult places for the United States to impose democracy in. Even though Rome did control Gaul for an extended period of time, it found its resources ever more taxed by the effort, and its ability to pay its troops after service more and more constrained. The US forces are already stretched with active conflict in two major countries.
Who will prove to be the "barbarians at the gate" for the American Empire remains to be seen. The echoes of a path long ago paved by the Roman Empire continue to emerge as we watch the whirlwind of the 'War on Terror' continuing.