While disparate peoples can be unified in the face of an external threat and inspired by bravery and sacrifice, such a confederation is in danger of falling apart (or, more significantly, becoming transformed) once the immediate threat is removed. Then, beware.
The hard-won unity that defeated the Persians, due in part to the heroic and inspiring warfare of the Spartans, soon collapses as Sparta goes its own way and Athens begins to take control of the other city-states through the Delian League. This estrangement soon leads to the Peloponnesian War between these two rival powers, and much built that had been beautiful was destroyed.
As for modern analogues, the United Nations comes to mind. Its purpose is for the world to be "united", but without a clear foe, "unity" devolves into disparate identities, ideas, and interests, all of which compete. Rather, somewhat like Athens, it keeps on building monuments to itself, although its monuments seem neither so grand nor as longlasting. The question then becomes, who will play Sparta to the Delian's Athens or Rome to the waning Greek hegemony?