In Parashat Chukkat, we suddenly find ourselves in the fortieth year in the wilderness. This is a period which witnesses the death of Miriam and Aaron. Moses, too, will be unable to enter the land due to his mishandling of yet another water crisis. Nor, does it seem, that the children of Israel have progressed from the cantankerous rabble that had left Egypt forty years earlier. They complain about water (twice), food, and the fact that they ever left Egypt!
However, in chapter 21 there are some remarkable developments that might pass us by if we fail to pay close attention to the text. The nation makes a vow before going to battle against Arad (21:2); they admit that they had sinned when they complained (21:7); they sing their own song of praise on receiving water without any prompting from Moses (21:17); and it is they (not Moses) who send the messengers to Sihon, King of the Emorites (21:21).
In each of these instances there is a suggestion that the nation has matured. A litmus test of maturity is the willingness to take responsibility and act independently. Until now, they had been largely dependent on Moses, even to the point that they only sang at the Red Sea in response to his singing. In the past they never admitted wrongdoing, but in the fortieth year they begin to.
The sages say that the age of 40 is that of understanding. After forty years of travelling and learning, the nation is ready to embark on the next important stage in its history of entering the Land of Israel and establishing a kingdom.