Argthtjtr

I'm looking for a way to describe the thing that finally triggers something after rising tensions. For example, some isolated incident triggers massive protests, but the protests are really due to decades of rising tensions.

The isolated incident itself is somewhat arbitrary and could have been one of many things, but the rising tensions were already ready to explode.

Preferably the expression wouldn't trivialize the triggering incident.

Any ideas?

The straw that broke the camel's back

and

The last straw

Grammarist:

The straw that broke the camel’s back refers to something seemingly
minor or trivial, that when added to a situation full of accumulating
difficulties, causes an extreme reaction or failure. The idea is one
of piling an extreme burden on a camel, until the weight of one, final
piece of straw becomes the tipping point that causes the camel to
collapse. References to the proverb of the straw that broke the
camel’s back may be found at the turn of the nineteenth century. Prior
to this time, the idea was found in a proverb that referred to a
feather that broke the horse’s back.

The last straw is an idiom that may be considered a sort of shorthand
for the proverb the straw that broke the camel’s back.

tipping point TFD

A critical or pivotal point in a situation or process at which some
small or singular influence acts as a catalyst for a broader, more
dramatic, or irreversible change.

As in:

After decades of simmering tensions, a tipping point was reached and demonstrations erupted.

This can also be known as a catalyst

Defined by Cambridge as:

a condition, event, or person that is the cause of an important change

As in:

Ultimately this report was the catalyst that caused the entire Bond Corporation to collapse. ¹

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