What caused Israel to act exactly as God predicted?
The conspiratorial power of Satan working behind the scenes caused Israel to act exactly as God predicted. Because they would not annihilate the nations, they got soft-hearted, spared some, made leagues with them, and commingled with their gods, committing spiritual adultery.
Les explains that God already knew Israel would not destroy the Canaanite nations. They would get soft-hearted and spare some, and the Gibeonites tricked them into making a league. Through this, Satan's conspiratorial power worked behind the scenes to cause Israel to do just what God said: forsake Him, break His covenant, and go whoring after the gods of the strangers of the land. Their commingling was spiritual adultery, which God hated and would judge. Today, the same principle applies when the true church commingles with other doctrines, such as the New Age movement.