In 390 BCE an army of Gauls, 30 thousand strong, marched out of northern Italy into Latium, an area that included Rome. They defeated a Roman army, sacked and burned Rome, and left only after being paid a large tribute. This marked a turning point for Rome, which resolved never again to allow such a disaster. Over the next century, Romans used a mixture of coercion and consent to bind their Italian allies more closely to them. Attempted secession was punished. But those who accepted their position as allies were not simply crushed and plundered (as in many other empires) but granted some or all of the privileges of Roman citizenship in return for military contributions. Membership in the Roman confederation was attractive enough that many Italian states sought it voluntarily.
The history of Greece during this period is different. Greek city-states never united. In the aftermath of the bloody Peloponnesian war, different city-states went on fighting for supremacy, until they were finally conquered by an outside power, Macedonia.
Peter Turchin is an ecologist-turned-social scientist who thinks that the contrast between Rome and Greece illustrates some general laws of history. According to Turchin, the rise and fall of empires is partly conditioned on the strength of “asabiya,” or social solidarity. (He borrows the term from the medieval Arab historian ibn Khaldun.) The strength of states depends not just on material factors like population size and wealth, but also on morale – on the willingness of citizens to work together for the common good (which includes punishing free-riders). Asabiya was high in early Rome; in Greece, by contrast, while individual city-states might evoke strong group feeling, there was little willingness to cooperate for the good of Greece as a whole.
Asabiya in turn (according to Turchin) develops especially along “metaethnic frontiers,” where very different cultures meet and clash. The illustration (by me, writing about matrilocal asabiya, not Turchin) shows the general idea. When culture changes little, or changes gradually, with distance (a), there is little basis for uniting independent polities (stars) into enduring larger units, and alliances (dotted lines) shift constantly. Along the metaethnic frontier (b), the opposite is true (solid lines show cohesive enduring units). Think Game of Thrones versus Lord of the Rings: it’s easier to get men and elves and dwarves to work together when they are fighting an army of orcs serving the Dark Lord.
Sometimes a metaethnic frontier develops where major religions or ideologies clash. But in the Roman case, the metaethnic frontier ran along the line dividing civilized Italians from barbarian Celts. Greece, by contrast, experienced a surge in fellow-feeling when Athens and Sparta fought together to defeat Persia, but this was too short lived to lead to a unified state.
Also worth reading is Empires of Trust, tracing parallels between the expansion of early Rome and of the United States – two immense states on the western frontiers of civilization. (The book is better than most comparing America and Rome.)