C. F. Keil and Franz Delitzsch on Judges 12:4

Jdg 12:4

He therefore gathered together all the men (men of war) of Gilead and smote the Ephraimites, because they had said, “Ye Gileadites are fugitives of Ephraim in the midst of Ephraim and Manasseh.” The meaning of these obscure words is probably the following: Ye Gileadites are a mob gathered together from Ephraimites that have run away; “ye are an obscure set of men, men of no name, dwelling in the midst of two most noble and illustrious tribes” (Rosenmüller). This contemptuous speech did not apply to the tribes of Reuben and Gad as such, but simply to the warriors whom Jephthah had gathered together out of Gilead. For the words are not to be rendered erepti Ephraim, “the rescued of Ephraim,” as they are by Seb. Schmidt and Stud., or to be understood as referring to the fact that the Gileadites had found refuge with the Ephraimites during the eighteen years of oppression on the part of the Ammonites, since such an explanation is at variance with the use of the word פּליט, which simply denotes a fugitive who has escaped from danger, and not one who has sought and found protection with another. The Ephraimites had to pay for this insult offered to their brethren by a terrible defeat.