Jud 11:29-31. HIS VOW.

      29, 30. Then the Spirit of the Lord came upon Jephthah--The calm wisdom, sagacious forethought, and indomitable energy which he was enabled to display, were a pledge to himself and a convincing evidence to his countrymen, that he was qualified by higher resources than his own for the momentous duties of his office.
      he passed over Gilead, and Manasseh--the provinces most exposed and in danger, for the purpose of levying troops, and exciting by his presence a widespread interest in the national cause. Returning to the camp at Mizpeh, he then began his march against the enemy. There he made his celebrated vow, in accordance with an ancient custom for generals at the outbreak of a war, or on the eve of a battle, to promise the god of their worship a costly oblation, or dedication of some valuable booty, in the event of victory. Vows were in common practice also among the Israelites. They were encouraged by the divine approval as emanating from a spirit of piety and gratitude; and rules were laid down in the law for regulating the performance. But it is difficult to bring Jephthah's vow within the legitimate range (see on Le 27:28).

JFB.