Parshat Pinchas begins with the aftermath of the Midianite seduction of Israelites. So God has some advice.
And Adonai spoke to Moses, saying: “Distress the Midianites, and strike them, for they distress you by their wiles with which they have beguiled you…”
Tzaror(distress). Like shamor (guard) and zachor (remember). This is language of present continuous action, therefore meaning you must remain hostile to them.