"Ephraim did not drive out the Canaanites who were living in Gezer, so the Canaanites lived in Gezer among them" (1:29). Jericho and other battles demonstrated that God could help Israel defeat any enemy. Yet, many of the tribes failed to complete the task of driving away all idol-worshippers from land they conquered. God had warned them about this. Idol-worship (and the corruption it included) was the dominant culture; Israel had to stay far from it.
Unfortunately, the result was predictable: "their gods will be a snare to you" (2:3). Sin is hard to resist when we are surrounded by it.