Which war was most important in establishing Rome as the greatest empire in the world?

Answer 1

I think it was the Punic Wars with Carthage. Rome won all three wars. The wars were fought for control of the Mediterranean Sea, Sicily, and Spain. When Hannibal crossed the Alps with a vast army he won victory after victory in Italy. Rome nearly died. But Rome prevailed by defeating him at the Battle of Zama in 202 B.C. The third war with Carthage saw the destruction and annexatio of Carthage by Rome. From this time on Rome was the dominant nation in Europe, Africa, and the Middle East. Of course there were other important conquests to come like Britain, Gaul, Greece, Egypt, etc. I hope this has been a help to you.

Answer 2

With the assassination of Julius Caesar in 44BC, Rome was plunged into a civil war which ended in victory for Gaius Octavius. In 27BC the Senate gave him the name Augustus, meaning exalted or holy one, and he became the undisputed Emperor.

Just one point, the Greatest Empire, if you're looking at size was actually the British Empire.