Summary: Gideon shunned position but kept power, his son Abimelech had position but not power as both were ruled by the flesh and from the flesh reaped corruption. Learn how we can fall prey to that as Christians and how to avoid it.
Galatians 5:16-17 (quickview)  But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do. ESV
We say we want to do God’s will, but when we let our fleshly nature have a place of primary importance for our values and actions, then we find ourselves doing and saying things we know we shouldn’t say or do.
Gideon did not want to be a king. He knew and even said that Yahweh should rule over Israel (which means "governed by God"). But in his actions he did exactly that - he became a king and let his flesh so rule that it was passed down to his son Abimelech who became a tyrant and was Gideon’s flesh times 10.
So these chapters begin a series of accounts that show us the result of the flesh. This is the story of power and position-what they do to us and the effects on others around us. Gideon and Abimelech are an interesting contrast. Gideon had influence (power) but shunned position-saying that God should be king, and yet went out and acted just like a worldly king of a Canaanite nation. Abimelech grabbed for position and got it, but lacked the influence (power) to hold on to it. Both are examples of what happens when we desire power or position instead of giving both to God.
Verses 1 - 3
We see here the beginning of the end of friendship between the tribes. Rivalries had been set aside for the most part as Israel conquered Canaan. But without Yahweh as their leader, the flesh starts to rear up and guide the thoughts of Ephraim.
Ephraim was a strong and proud tribe. They’d helped Ehud and Barak and were now insulted that they hadn’t been included in the fight with Gideon and his "puny" group of 300 soldiers. May I just say that God doesn’t need a massive, well organized group with a history in order to do His will? He can use 300 rag tag solders with some torches and jars. He can use you, even though you don’t have all the experience or credibility in the world to do His will.
Anyway - Gideon is very smart in appealing to Ephraim’s pride in appeasing them. He basically convinces them that they are going to get the bigger victory by going after the Midian princes Oreb and Zeeb.
The animosity towards Gideon doesn’t end here, though but for the opposite reason.
Verses 4 - 21
Succoth and Peniel (which are near one another) complain because they want no part in Gideon’s battle. It’s thought that the reason was they believed that the Midianites would eventually win and if they sided with Gideon now they would regret it later. It shows that their loyalties have switched from a spiritual loyalty to God, to a political loyalty based on how they can fare best.
Let me just say that this is a dangerous attitude to take - but one many Christians adopt all too easily. How does this happen? It can be subtle. Let me use an example that’s sure to rankle someone’s feathers: homosexuality. The Bible makes it abundantly clear that homosexuality does not reflect the character of God and is thus sin - along with lying, murder, and a hundred other things.