The Sicilian Expedition
At the height of its long war with Sparta, Athens gambled on an audacious expedition to conquer Syracuse, a wealthy Greek city in distant Sicily — and lost almost the entire fleet and army, tens of thousands of men, in a catastrophe it never recovered from. It was a self-inflicted wound, born of overconfidence and greed for more. Athens had reached past the line it could supply and hold, and never recovered the power it spent there.
A state's commitments outrun the resources and logistics needed to hold them; the margin fails first.