After the conquest of Canaan, God deliberately left certain nations in the land — not because He couldn't remove them, but to test Israel (Judges 3:1-2). The rabbis found this practice disturbing enough to need explanation. A psalmist had already asked for it: "Do not kill them, lest my people forget; make them totter by your power" (Psalm 59:12). Left without enemies, Israel forgets. Given enemies, Israel remembers who it needs.
The midrash then traces the angel Michael through the wilderness narrative: he appeared to Moses in the burning bush (Exodus 3:2), guarded the camp at the sea (Exodus 14:19), and stands throughout as Israel's divine protector. But the rabbis make a distinction: Michael guards, God leads. When Moses refused the angel-as-intermediary and demanded God's direct presence — "If your presence does not go with us, do not bring us up from here" (Exodus 33:15) — God agreed. No substitute. No proxy. The covenant demanded the presence itself, not its representative.
This is the heart of Israel's test: not whether they can survive enemies, but whether they can remember the difference between God's presence and God's representative. Nations can be kept in check by angels. Only Israel demands — and receives — the thing itself. The nations left in the land are not God's failure to complete the conquest. They are the ongoing conditions for Israel's faithfulness.