What caused Israel to do exactly what God predicted they would do?
The conspiratorial power of Satan working behind the scenes caused Israel to do just what God said they would do. This involved sparing some inhabitants, being tricked by the Gibeonites, and commingling, which was spiritual adultery and breaking God's covenant.
Les explains that God predicted Israel would not annihilate the inhabitants of the land. Instead, they would get soft-hearted, spare some, and be tricked by the Gibeonites into making a league with them. By hook or by crook, the conspiratorial power of Satan working behind the scenes caused Israel to do exactly what God said. This commingling was spiritual adultery, and God would judge it, kindling His anger, forsaking them, and hiding His face, as stated in the song that would testify against them.