Breaking apart into small pieces; falling into decay or disrepair.
From Middle English 'crumble' (possibly from Old English 'cruma' meaning crumb, or Germanic roots). The '-ing' form shows the ongoing process of breaking apart.
Crumbling is fascinating because it describes something happening in real-time—an empire doesn't 'crumble' if it's already destroyed; the word specifically captures that middle moment when things are falling apart, which is why it's used so often in literature about decline.
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