נקם נקמת בני ישראל מאת המדינים... וישלח אתם משה... ואת פנחס בן אלעזר
Hashem instructed Moshe Rabbeinu to exact revenge against the Midyanim for their role in causing Klal Yisrael to sin with the Moavite girls and worship the Peor idol. Moshe himself did not lead the way; instead, he chose Pinchas. Rashi attributes Moshe’s reasoning to the fact that Pinchas had begun the deed of reckoning, by slaying Kozbi, the Midyanite Princess, who had cohabited with Zimri, the renegade Prince of the Tribe of Shimon. Let the one who initiates the revenge carry on to the next phase. Alternatively, Pinchas was a descendant of Yosef HaTzaddik who was sold by his brothers…