Was Rome really ever much of an empire?!? Pullease. :^)
Rome's first conquest was Ostia, circa 400 BC; the empire was divided by the Emperor Diocletian before 305 AD; the eastern half of the empire reconquered a good bit of the fallen western half of the empire during the reign of Justinian, but regardless, endured with rising and falling territorial fortunes until the Turks finally waltzed on in and took Constantinople in 1453 -- iow, a span of at least 1853 years.
Although Rome never bothered to conquer Hibernia (allegedly modern Ireland) or northern Calendonia (allegedly modern Scotland, the Scots didn't arrive from Ireland until the 7th century AD), in each case because there wasn't anything worth conquering, Roman commerce was always far-reaching, even after the 5th c fall of Rome.
The full extent of Roman conquest is still unknown, but apparently Rome established a base or trading post on the coast of Ireland (there's an FR topic of course), and within the past ten years or so an archaeological rescue dig turned up a Roman cemetery in Copenhagen, Denmark.
I’ve heard that “There was nothing worth conquering” bit before about Ireland and Scotland, yet when one looks at the effort and maintenance required for Hadrian’s wall it is very clear to any impartial person that Rome simply couldn’t conquer the Scots. To build and staff the fortifications certainly was a drain, and it could have been avoided or minimized by simply marching to the northern coast. Even the nuisance raids from the Irish don’t seem to be something Rome tolerated from other neighbors; while Rome probably had the military might to defeat both of these places, they certainly didn’t have it in the right place and time to do so.
Is it possible that Rome never bothered to conquer Caledonia because, as Caesar was reported to have said:
“Caledonia, Quod facit caput tuum magnum et difficile?”