Rashi on Deuteronomy 32:42/רש"י על דברים ל״ב:מ״ב

1אשכיר חצי מדם I WILL MAKE MY ARROWS DRUNK WITH BLOOD of the enemy,

2וחרבי תאכל AND MY SWORD SHALL DEVOUR their FLESH.

3מדם חלל ושביה FROM THE BLOOD OF THE SLAIN AND THE CAPTIVE i.e., this will happen to them because of the sin of those slain in Israel (i.e., on account of their blood; מדם), and because of the captives they have made amongst them,

4מראש פרעות אויב — because of the sin of even the very first (ראש) breach (פרעות) made by the enemy (פרצות = פרעות); because when the Holy One, blessed be He, exacts punishment from the heathen nations for the wrongs they have inflicted upon Israel, He visits on them their own sin and the sins of their ancestors, from the very first breach they have made in Israel (Sifrei Devarim 332:5).