וַֽיִּצְבְּאוּ֙ עַל־מִדְיָ֔ן כַּאֲשֶׁ֛ר צִוָּ֥ה יְהוָ֖ה אֶת־מֹשֶׁ֑ה וַיַּֽהַרְג֖וּ כָּל־זָכָֽר׃
And they warred against Midian, as the LORD commanded Moses; and they slew every male.
Read kabbalistic (mystical) interpretation of Numbers 31:7: and classical mystical sources.