Harass the Midianites and strike them.
Tanchuma Pinchas 3
English: “Harass the Midianites.” Why? “Because they are harassing you.” From here our Rabbis of blessed memory said: if one comes to kill you, rise early to kill him.
Halakhic principle derived from the Midian command.
Imminent lethal threat licenses preemptive self-defense.