Doji did a good job of explaining.
I would add that my "throwing the first punch" represents the actual start of violence.
Instigating would be any action short of violence whose purpose was to make the situation elevate to violence.
It might not be a "punch"; it could easily be a knife, a shillelagh (an Irish fighting cudgel), a gun or a howitzer, or any other way of elevating the conflict to violence.
I should note that there are laws about inciting to violence--the so called Brandenburg ruling.
Brandenburg v. Ohio - Wikipedia
The Court held that government cannot punish inflammatory speech unless that speech is "directed to inciting or producing imminent lawless action and is likely to incite or produce such action."
[2][3]:702 Specifically, the Court struck down Ohio's criminal syndicalism statute, because that statute broadly prohibited the mere advocacy of violence.
