European diplomacy since the start of the high middle ages has been defined by the idea of ‘balance of power’. No one state should ever grow powerful enough that it could take over everything. For example, almost all of Europe was united time and again against France under Napoleon not because he posed an immediate threat to all of Europe at once, but because with each conquest of a single small country the next one fell easier, until by the time he attacked the big countries like the UK it would be too late. People had that foresight to band together once it became clear their common foe could grow out of hand. Yet when Carthage fell to Rome, the Hellenistic Kingdoms barely complained. When Alexander conquered Persia, no one thought they could be next. When Julius Caesar conquered Gaul, it’s like the Germans and Britons never even considered that they could be next on the chopping block. Rome fought and crushed each of her foes one by one, they never united in the face of terminal danger. There was no ‘containment war’. There was no ‘anti-roman coalition’. No Hellenistic Kingdom contacted the barbarians of the west to help them. Why did that all change after the fall of rome?
The Classical Era is filled with coalitions and defensive pacts. The so-called "Delian League" was originally meant to serve as a pan-Greek coalition against future Persian aggression. That coalition devolved (or evolved) into the Athenian "empire", which by the end of the century was aligned against and at war with the rival Peloponnesian league, headed by the Lakedaimonians. Virtually all of the belligerents in that war were aligned with one or the other, and the ones who tried to remain out of these larger structures were often destroyed (Melos, e.g.).
Other political coalitions of the Classical Era included the Ionian League, or the Achaean League. From literature, the "1,000 ships" which launch under the nominal authority of Agamemnon to assault Troy.
On the Roman end: early Italia was in a constant state of flux between different webs of alliances, normally with Rome either at odds with them or forming them. The Etruscan cities operated under a loose confederacy, and much later the Italian "allies" banded together in a new super-political push to rise up against the Romans in 91-90 BCE.
In the religious sphere, there were the amphictyony organizations which consisted of member cities near religious cites, like Delphi. They were intended to regular the sanctuaries and the events surrounding them, or impacting them.