FROM A READING OF JUDGES 1-2, it appears that after the initial Israelite victories, the pace of conquest varied considerably.
In many cases tribes were responsible for bringing their own territories under control. With the passage of time, however, it seems to have become unstated policy, as the Israelites grew stronger, not to chase the Canaanites from the land, nor to exterminate them, but to subjugate them or even enslave them, to make them "drawers of water and hewers of wood," to subject them to forced labor (Judg. 1:28).
The inevitable result is that a great deal of paganism remained in the land. Human nature being what it is, these false gods inevitably became a "snare" to the covenant community (Judg. 2:3). Angry with their refusal to break down the pagan altars, the angel of the Lord declares that if the people will not do what they are told, he will no longer provide them with the decisive help that would have enabled them to complete the task (had they been willing!). The people weep over the lost opportunity, but it is too late (Judg. 2:1-4). It is certainly not that they had never been warned.
This is the background to the rest of the book of Judges. Some of its main themes are then outlined for us in the rest of chapter 2. Much of the rest of the book is exemplification of the thinking laid out here.
The main thrust, shot through with tragedy, is the cyclical failure of the covenant community, and how God intervenes to rescue them again and again. Initially, the people remained faithful throughout Joshua's lifetime and the lifetime of the elders who outlived him (Judg. 2:6). But by the time that an entirely new generation had grown up — one that had seen nothing of the wonders God had performed, whether at the Exodus, during the wilderness years, or at the time of the entrance into the Promised Land — fidelity to the Lord dwindled away. Syncretism and paganism abounded; the people forsook the God of their fathers and served the Baals, i.e., the various "lords" of the Canaanites (Judg. 2:10-12). The Lord responded in wrath; the people were subjected to raids, reversals, and military defeats at the hands of surrounding marauders. When the people cried to the Lord for help, he raised up a judge — a regional and often national leader — who freed the people from tyranny and led them in covenantal faithfulness. And then the cycle began again.
And again. And again.
Here is a sober lesson. Even after times of spectacular revival, reformation, or covenantal renewal, the people of God are never more than a generation or two from infidelity, unbelief, massive idolatry, disobedience, and wrath. God help us.
Judges 2; Acts 6; Jeremiah 15; Mark 1