Judges chapter 2 chronicles how Israel quickly spiraled out of control when the generation that entered the Promised Land died–the generation of Joshua. They did evil in the sight of the Lord and worshiped other gods, gods that their neighbors worshiped. Did the previous generation taught them the commandments, regulations and decrees of the Lord their God as Moses told them? Or we’re they so busy conquering and dividing the land that they neglected to teach the children the way that they should go?
Because of this neglect, the current generation did not have a strong enough conviction to worship the true and living God. The consequence was bad. The nations that they did not conquer during the lifetime Joshua (the Jebusites, Canaanites, Hivites, etc.) became a stumbling block for them. Until God raised judges to guide them they would be raided or pillaged. The Israelites could not stand up to them. But when these judges died, the Israelites returned to their evil deeds.
What went wrong? The Lord spoke to them. Angels spoke to them. Was that not enough proof of God?
It is really important to pass on to our children the Word of God and remain steadfast in our faith. When God makes a pledge, he will stand on that promise. He will be faithful. It is us who become unfaithful to the promise.
God does not break His promises, we do.