Your event may be the corona virus, which the president continues to poo poo. As to the military breaking into militias, that is sort of happening already. Some of the III Percent Militia are both active and retired police and military, mostly retired.
How a war starts, well we sort of already are in a cold phase of one, like Blood Kansas. All these mass shootings and the organizing in the Far Right has those echoes. I have made the case of a cold civil war as well, and all hot wars do not start hot.
It gives me no comfort by the way.