External threats help, but unification was often, ahem, self-promoting as well. A hundred years on, becoming Roman by force and becoming Roman to avoid becoming Persian was kind of indistinguishable - and both did happen.

We're kind of in uncharted territory with today's ideologically-driven democracies, but I'd argue the EU itself is a kind of peaceful equivalent. New members accrue because the economic and other benefits vastly exceed whatever beef it's neighbors have with it.