C. F. Keil and Franz Delitzsch on Judges 11:1

Jephthah Elected as Prince; Negotiations with the Ammonites; Victory, Vow, and Office of Judge - Judges 11-12:7On the nature of the sources from which the author drew this tolerably elaborate history of Jephthah, all that can be determined with certainty is, that they sprang from some contemporary of this judge, since they furnish so clear and striking a picture of his life and doings. Bertheau's hypothesis, that the section extending from Jdg 11:12 to Jdg 11:28 is founded upon some historical work, which is also employed in Num 21; Deut 2:1-3:29, and here and there in the book of Joshua, has really no other foundation than the unproved assumption that the Pentateuch and the book of Joshua were written towards the close of the period of the kings. For the marked agreement between Jephthah’s negotiations with the king of the Ammonites concerning the possession of the land to the east of the Jordan, and the account given in the Pentateuch, especially in Num 20-21, may be explained very simply and very perfectly, on the supposition that the author possessed the Pentateuch itself. And the account which is wanting in the Pentateuch, namely, that Israel petitioned the king of Moab also for permission to go through his land (Jdg 11:17), may have bee added from oral tradition, as those glorious victories gained by Israel under Moses were celebrated in verse by contemporaneous poets (see Num 21:14, Num 21:17, Num 21:27); and this certainly contributed not a little to keep alive the memory of those events in the nation for centuries long.

Election of Jephthah as Prince and Judge of Israel. - Jdg 11:1-3. The account begins with his descent and early mode of life. “Jephthah (lxx Ἰεφθά) the Gileadite was a brave hero” (see Jdg 6:12; Jos 1:14, etc.); but he was the son of a harlot, and was begotten by Gilead, in addition to other sons who were born of his wife. Gilead is not the name of the country, as Bertheau supposes, so that the land is mythically personified as the forefather of Jephthah. Nor is it the name of the son of Machir and grandson of Manasseh (Num 26:29), so that the celebrated ancestor of the Gileadites is mentioned here instead of the unknown father of Jephthah. It is really the proper name of the father himself; and just as in the case of Tola and Puah, in Jdg 10:1, the name of the renowned ancestor was repeated in his descendant. We are forced to this conclusion by the fact that the wife of Gilead, and his other sons by that wife, are mentioned in Jdg 11:2. These sons drove their half-brother Jephthah out of the house because of his inferior birth, that he might not share with them in the paternal inheritance; just as Ishmael and the sons of Keturah were sent away by Abraham, that they might not inherit along with Isaac (Gen 21:10., Gen 25:6).