Exodus 23:29
I will not drive them out from before thee in one year; lest the land become desolate, and the beast of the field multiply against thee. I will not drive them out from before thee in one year; lest the land become desolate, and the beast of the field multiply against thee.
God would not drive the inhabitants out from before Israel in one year or all at once, but would displace the Canaanite nations gradually, little by little. That gradual driving out is taught as the meaning of the promise not to act in a single year. The reason given is to keep the land from becoming desolate and empty and to keep the wild animals of the field from multiplying against Israel. Leaving the land empty would have exposed it to desolation and to increase of beasts against a people too few to hold it. The gradual conquest would therefore continue as Israel grew in number until the people had increased enough to inherit and occupy the land.
The promise belongs to God's giving of His commandments and call to obey, with the promised blessing depending on Israel's obedience. Israel pledged to comply, but its idolatry showed disloyalty and brought judgment. The chapter presents Moses as carrying God's truth, light, and grace into practice before the people.