Summary: After the death of Deborah, a reversion to idolatry triggered a period of Midianite oppression. Gideon was appointed to combat both the physical threat and the spiritual corruption.
A deeper look: In an act reminiscent of Abraham, Gideon smashed his own father’s idols together with those belonging to other Jewish families. Gideon now needed to raise an army to fight Midian and Amalek. Yet for a Jewish army to be effective, its heart must be as strong and true as its sword. God told Gideon to bring prospective recruits to drink from a watering hole and to separate those who lapped the water with their hands from those who kneeled to drink. Rashi (d. 1105) explains that those who kneeled were used to doing so for idol worship, rendering them unsuitable to fight.
Gideon hears someone relating a dream that barley bread was roasting in the Midianite camp. Rashi notes that the barley bread symbolised a type of mincha (meal) offering called the omer which is made from barley and brought on the second day of Pesach (Leviticus 23:9). Whearas other meal offerings are offered by an individual (Leviticus 2:1), the omer is a communal offering. Rabbi Meir Simcha of Dvinsk (d. 1926) elucidates the symbolism of the dream: Midian cannot be defeated by individuals but by a people joined together as one unified community (Meshech Chochma, Haftarah Shabbat Hagadol).
After executing the Midianite kings, Gideon removed ‘the crescents that were on the necks of their camels’ (Judges 8:21). Interestingly, Rabbi David Kimchi (d. 1235) notes that the Midianites were descendants of Ishmael (Radak loc. cit. 24). After gathering other jewellery, Gideon fashioned a replica of the breastplate worn by the High Priest and hung it in the city.
Rabbi Moshe Alshich (d. 1593) explains that the Ephod specifically atoned for the sin of idol worship and Gideon hoped to inspire the people to abandon the worship of foreign deities and instead worship God. Yet after his death, through a turn of tragic irony the very symbol fashioned to inspire the people against idolatry became an object of worship, triggering another cycle of idolatry and suffering.