‘And so it was that, as soon as Gideon was dead, the children of Israel turned again, and went a-whoring after the Baalim, and made Baalberith their god.'
This is illustrated further in Judges 9. It was partially the result of his many wives, as Judges 9 demonstrates. Baal-berith, ‘lord of the covenant', was the Shechemite god. He is probably to be equated with El-berith, ‘god of the covenant' (Judges 9:46). The Shechemites are later called ‘the sons of Hamor (the ass)'. At Mari the ass was associated with covenant making. Seemingly among the Amorites a covenant had to be sealed by the sacrifice of an ass. The same seems to have applied here with the Shechemites in their covenant with Baal-berith.
There is no record of Shechem ever being captured by Joshua and it may be that their worship of ‘the lord of the covenant' had convinced Joshua that they were true worshippers of Yahweh so that they were welcomed to participate in the covenant ceremony at Shechem (Joshua 24). Indeed Yahweh and Baal-berith may well have been equated. But if so this had now degenerated back to worship with Baalistic tendencies under that name. The result was that the accession of Abimelech led to the children of Israel again turning to Baal worship.