The word I am searching for is used in the field of International Relations. It is used to describe a specific event that causes an ongoing passive conflict to escalate into active conflict. It is similar to, but is not the word 'catalyst'.
The term indicated an event triggering a direct change from non-violent to violent action.
For example: The use of beef grease (tallow) in cartridges as the spark to the Indian Rebellion of 1857.
The use of tallow in rifle cartridges [term] the Indian Rebellion of 1857.
In the case of Sri Lanka the two peoples had lived together under British rule but following their departure the desire by some Sinhalese to assert their culture as dominate over another was brought into the public sphere of politics by the Sri Lanka Freedom Party and became a [term] for violence to begin.