Numbers 33:51

Speak unto the children of Israel, and say unto them, When ye are passed over Jordan into the land of Canaan;

The crossing of the Jordan into Canaan was the appointed moment when the instructions for entering and possessing the promised land were to be carried out. At that moment the command was to drive out the inhabitants of Canaan and to take possession of the land given to Israel. With that they were to destroy images and idols, tear down high places, and remove all objects and monuments of heathen worship.

That removal was required because those objects and the influence of the people who used them could draw Israel into idolatry and lead them away from God. Fear, laziness, self-interest, or human ideas of mercy were not to be allowed to override obedience to that instruction. The warning was afterward fulfilled, for the inhabitants left among them became a lasting trouble to Israel.