the.com/powder keg

means A situation or place so tense and unstable that almost anything could trigger a sudden, explosive crisis.

from Literal before it was figurative: a powder keg was a wooden barrel used to store and transport gunpowder, common on ships and in armories and forts from around the 1600s onward. A stray spark near one meant catastrophe, so the phrase jumped into political and social language by the 1800s to describe any place or situation — a tense region, a heated argument, a crowded room of grudges — primed to blow at the smallest provocation.