The Fall of Western Rome
In 476 CE a Germanic general deposed the last emperor of the western Roman Empire — a boy named, almost too neatly, Romulus after Rome's founder — and simply left the throne empty. There was no single catastrophe; the west had been hollowing for centuries under overstretched borders, a broken tax base, migrations it could no longer absorb, and its own institutional decay. It is history's defining case of imperial overstretch: commitments that finally, permanently outran the means to hold them, ending less with a bang than a shrug.
A state's commitments outrun the resources and logistics needed to hold them; the margin fails first.