‘And he gathered to him the children of Ammon and Amalek, and he went and smote Israel and possessed the city of the palm trees.'
This city of the palm trees must have been Jericho, for the champion raised up was a Benjaminite. Thus the territory of Reuben and Gad was affected, and a part of Benjamin. Whether it was Yahweh Who arranged the confederacy, or the king of Moab, or both, the result was the same. Ammon and Moab were situated side by side in Transjordan and regularly acted together, for they were brother tribes, so much so that they had jointly come under Yahweh's judgment (Deuteronomy 23:3-6). Amalek were at least partly Bedouin and fairly widespread.
“And went and smote Israel and possessed the city of the palm trees.” Presumably first the Reubenites and part of Gad, and then part of Benjamin. It probably did not affect the other tribes. Jericho had not been rebuilt, but there may have been a temporary settlement on it, or a guardpost. But the surrounding area was very attractive. It guarded the Jordan crossing. This was presumably the outer limit of their depredations.
Why then did the tribal confederation not come to their aid? They may themselves have been involved with their own protection against marauding enemies and unable to leave their own area. Or it may simply indicate a weak period in the tribal confederacy when they were not prepared to do so because of the weakness of their dedication to Yahweh. Possibly the threat was not seen as too great compared with other threats. The Philistines themselves probably presented a constant greater threat as they sought to expand their newly won territories.