The end of the desert wanderings and the arrival in the Land of Israel marked the beginning of a new chapter for the Israelites, but it also introduced a new phase of spiritual failures. The primary approach among commentators is that this period refers to the days of Joshua and the initial settlement of the land. The central failure during this time was the refusal to completely eliminate and drive out the local inhabitants. God had given an explicit command not to leave a single soul alive and to prevent the local nations from remaining in the land. However, the Israelites chose a different path. Instead of carrying out His instruction, they preferred to let many of the local people live, turning them into tax-paying subjects [רש״י, מאירי].
Ignoring God's direct command brought an earlier divine warning to life. He had cautioned that any nations left behind would become a dangerous trap and a heavy burden. Indeed, by allowing these inhabitants to stay, the Israelites eventually mingled with them. They adopted the local customs, which led to a severe moral decline involving idol worship and bloodshed [מלבי״ם, מאירי].
Looking at this from a wider historical angle, this early failure takes on an additional layer of meaning. It serves as part of a prayer spoken by the Israelites later living in exile. By recalling this history, they remind God that even though their ancestors violated His direct command the moment they entered the land, He still did not abandon them entirely [אלשיך].