A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jdg 12:1THE EPHRAIMITES QUARRELLING WITH JEPHTHAH. (Jdg 12:1-3)
the men of Ephraim gathered themselves together--Hebrew, "were summoned."
and went northward--After crossing the Jordan, their route from Ephraim was, strictly speaking, in a northeasterly direction, toward Mizpeh.
the men of Ephraim . . . said unto Jephthah, Wherefore . . . didst [thou] not call us?--This is a fresh development of the jealous, rash, and irritable temper of the Ephraimites. The ground of their offense now was their desire of enjoying the credit of patriotism although they had not shared in the glory of victory.
jdg 12:2when I called you, ye delivered me not out of their hands--The straightforward answer of Jephthah shows that their charge was false; their complaint of not being treated as confederates and allies entirely without foundation; and their boast of a ready contribution of their services came with an ill grace from people who had purposely delayed appearing till the crisis was past.
jdg 12:3when I saw that ye delivered me not, I put my life in my hands--A common form of speech in the East for undertaking a duty of imminent peril. This Jephthah had done, having encountered and routed the Ammonites with the aid of his Gileadite volunteers alone; and since the Lord had enabled him to conquer without requiring assistance from any other tribe, why should the Ephraimites take offense? They ought rather to have been delighted and thankful that the war had terminated without their incurring any labor and danger.
jdg 12:4DISCERNED BY THE WORD SIBBOLETH, ARE SLAIN BY THE GILEADITES. (Jdg 12:4-15)
the men of Gilead smote Ephraim, because they said, Ye Gileadites are fugitives of Ephraim--The remonstrances of Jephthah, though reasonable and temperate, were not only ineffectual, but followed by insulting sneers that the Gileadites were reckoned both by the western Manassites and Ephraimites as outcasts--the scum and refuse of their common stock. This was addressed to a peculiarly sensitive people. A feud immediately ensued. The Gileadites, determined to chastise this public affront, gave them battle; and having defeated the Ephraimites, they chased their foul-mouthed but cowardly assailants out of the territory. Then rushing to the fords of the Jordan, they intercepted and slew every fugitive. The method adopted for discovering an Ephraimite was by the pronunciation of a word naturally suggested by the place where they stood. Shibboleth, means "a stream"; Sibboleth, "a burden." The Eastern tribe had, it seems, a dialectical provincialism in the sound of Shibboleth; and the Ephraimites could not bring their organs to pronounce it.
jdg 12:7Jephthah died--After a government of six years, this mighty man of valor died; and however difficult it may be for us to understand some passages in his history, he has been ranked by apostolic authority among the worthies of the ancient church. He was followed by a succession of minor judges, of whom the only memorials preserved relate to the number of their families and their state [Jdg 12:8-15].