Thomas Byrne in an e-mail that continues the discussion of the U.S. empire compares the conflict in 1861–1865 to the Samnite Wars. Rome secured Italy in the Samnite Wars (343–290 B.C.) that involved "almost all the states of Italy." It thus became an empire that could dominate all of Italy and extend further thereafter. Byrne notes:
"I have long thought that slavery and tariffs were secondary causes of the conflict in 1861. It was the Empire securing its local area before expanding overseas. (Much like the Roman-Samnite wars for instance.) After subduing the South, the empire turned westward and destroyed the Indian 'threat'. Soon after we would turn to Cuba and overseas conquests."
I think this explanation of the war has strong legs. It fits in with the fact that Lincoln represented the fast-growing western states, and it explains the subsequent continental expansion.
It's not a complete explanation of why the drive for a state to become an empire occurs where it occurs and when it occurs. We do not understand what conditions may contribute to this drive over the broad scope of human history in which there have been so many empires. Since slavery and tariffs are particular matters associated with the expansion of empire in this country, they can't explain the many empires where those factors are absent, including the Roman case. The simplest theory may be the elimination of rivals that otherwise would consolidate power and become a serious threat. The article on the Samnite Wars suggests that factor. It's obvious that secession of the Southern states was a threat to the existing U.S. empire's breadth and durability in 1861 since it directly reduced its size and opened up the western region to a competitive force.