So, it was the military, as you wonderful gents had so wisely posited. For future reference: forbidding and then reclaiming claimed food does nothing productive. The fort crashed into a tantrum spiral, leaving many dwarves to throw themselves into one of several open magma pits. The fort might have survived, but one partcularly angry dwarf decided to open the bridge, letting in the hoard of goblins and trolls awaiting outside, and to release the cyclopse from his prison. The military fought valianly on both fronts, but they had sub-standard equipment. They did, however, manage to kill the semi-megabeast. The goblins proceeded to murder every last dwarf, dwarfette, and dwarfling, save a single legendary metalcrafter, whom I managed to sequester down in the forges. This anti-social dwarf could not care less that everyone he lived next to for the past three years had just died a horrible, painful death. Luckily, he was locked in with all of the metal bars I had ever made, so I set him to work building giant axe blades and so forth.
Most unfortunately, an opportunistic fire imp took it upon himself to topple this fort forever by melting the face of off my survivng dwarf. He bled for several tiles before finally succumbing to his wounds. What fun.
I on't plan on starting my next fort until the new release, but Arkham City and Skyward Sword should keep me occupied until then. Skyrim also looms on the horizon of possibility (which would make a good fort name, come to think of it).