Judges 12:2

And Jephthah said unto them, I and my people were at a great
strife with the children of Ammon
As to the cause of the war, or the reason of his going over to fight the children of Ammon, it was a strife or contention between the Gileadites and them, concerning their country; which the children of Ammon claimed as theirs, and the Gileadites insisted on it they had a just right to it; by which it appeared that this was not a personal contention between Jephthah and them; and therefore the Ephraimites had no reason to fall so furiously upon him particularly; and it was a contention which chiefly concerned the two tribes and a half, and not the rest; and so could not be blamed for defending themselves alone if they could, without interesting others in the quarrel: but this is not all he has to say, he adds,

and when I called you, ye delivered me not out of their hands;
it seems he had called them to assist in driving the enemy out of their boarders when there, and they refused to help him; though it is not elsewhere said, and it is not denied by them, so that it was false what they alleged; or however, since they declined giving him any assistance, when the children of Ammon were in his country, he could not expect they would join him in an expedition into theirs.