“When you transgress the covenant of YHWH your God, which he commanded you, and go and serve other gods, and bow down yourselves to them, then the anger of YHWH will be kindled against you, and you will perish quickly from off the good land which he has given you.”
All outwardly depends on obedience to His covenant. In that covenant they have bound themselves to Him as their Supreme Sovereign, and have sworn to love and serve Him and to obey His commandments. They have agreed only to offer sacrifices at the central sanctuary, or at places where God was pleased to record His name and reveal Himself, and to gather at the covenant festivals to worship Him. They have sworn to abjure idolatry and the ways of the Canaanites. But if they turn aside from this and seek and serve other gods, then they will have broken their covenant vows, and YHWH's just anger will be roused and He will bring judgment on them. The covenant will lose its effect. They will be removed from the land.
Thus were covenant love and covenant obedience bound closely together. In the end God's promises to Abraham were unconditional. He would bring about their fulfilment. But delay was possible through disobedience and during that process those who fell short would suffer the due reward for their disobedience. Yet as we see later in Judges, His wrath was tempered with mercy. If they repented then He would again act for them and deliver them from the position in which they had placed themselves.