Fill in the blank: American militias began to change the game and created their own rules using (blank) style warfare which included
ambush, sabotage and raids.
Your answer

Guerilla

Explanation:

Guerilla style warfare is a type of unconventional warfare in which a group of aggressors, mostly military or rebels makes use of ambushes, sabotage, raids, and other fighting tactics to fight and try to subdue a much larger force.

Therefore, the complete sentence goes thus: American militias began to change the game and created their own rules using guerilla-style warfare which included ambush, sabotage, and raids.