The End of the British Empire
Bankrupted by two world wars and confronted by mass movements demanding freedom, Britain let go of the empire on which the sun had famously never set — starting with India in 1947 and accelerating after the humiliating Suez crisis of 1956 proved it could no longer act as a world power alone. Within two decades, dozens of new nations were born. It is imperial overstretch meeting a collapsing justification — Rome and Spain's exit from history, on a 20th-century timetable.
A state's commitments outrun the resources and logistics needed to hold them; the margin fails first.