Life in the wilderness was far from plain sailing. The elements were unfriendly. Food was rationed. Water was scarce. The people were full of grumbles. The local tribes were hostile. It is amazing that Moses was so successful in keeping his cool.
One of the external difficulties was the need to fight against the desert peoples.
An example is the attack of “the Canaanite, the king of Arad, who dwelt in the south”. He “heard that Israel had come by the route of Atarim, and he fought against Israel and took some of them captive” (Num. 21:1).
There must be some special significance in the reference to Atarim. We know of a place called Atarot, indeed several such places (e.g. Num. 32:3), but this is the only Biblical reference to Atarim. Brown, Driver and Briggs’ lexicon suggests that it indicates a caravan route.
The traditional Jewish view does not understand it geographically as much as historically. Noting that the story is preceded by the episode of the twelve spies who were sent to check out the land, the sages link Atarim with ha-tarim, “the spies”, and say “the route of Atarim” could mean “the way of the spies”.
Possibly the king of Arad had a good intelligence service which followed the movements of the spies and saw that their largely pessimistic report shattered the morale of the people of Israel. Seeing how low the Israelites now felt, this was the time to attack.
A people that had lost faith in its God and itself was unlikely to be able to repel an attack.