Imagine winning a huge battle, but leaving a few enemy soldiers behind. They might seem insignificant at first, but what happens later?
Joshua is looking back at the incredible victories God has granted Israel. They've driven out many nations and conquered vast territories. But here, he reveals a crucial point: God isn't going to finish the job of clearing out all the remaining peoples.
This isn't a sign of God's weakness, but a test for Israel. By allowing these nations to remain, God is creating a situation where Israel's faithfulness will be constantly challenged. The incomplete victory is a setup for a future struggle of obedience, not military might.