According to the author of the book, this verse tells us that for eighteen years, the people of Israel were bothered and treated badly by others who lived in a place called Gilead. It's like when bullies were being mean to them for a long, long time.
Judges 10:8: And that year they vexed and oppressed the children of Israel: eighteen years, all the children of Israel that [were] on the other side Jordan in the land of the Amorites, which [is] in Gilead. [oppressed: Heb. crushed]
Jesus instructs to love your enemies, which contradicts the idea of conflict and oppression described in Judges 10:8.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;