John Gill Commentary Judges 12:2

"And Jephthah said unto them, I and my people were at great strife with the children of Ammon; and when I called you, ye saved me not out of their hand." — Judges 12:2 (ASV)

And Jephthah said unto them, I and my people were at a great
strife with the children of Ammon
The cause of the war, or the reason he went to fight the children of Ammon, was a strife or contention between the Gileadites and them concerning their country. The children of Ammon claimed it as theirs, while the Gileadites insisted they had a just right to it. This showed it was not a personal contention between Jephthah and them, so the Ephraimites had no reason to attack him so furiously in particular. It was a contention that chiefly concerned the two and a half tribes, not the rest, so they could not be blamed for defending themselves alone if they could, without involving others in the quarrel. But this is not all he says; he adds,

and when I called you, you delivered me not out of their hands ;
It seems he had called them to assist in driving the enemy out of their borders when they were there, but they refused to help him. Though this is not said elsewhere, and they did not deny it, making their allegation false.

However, since they declined giving him any assistance when the children of Ammon were in his country, he could not expect them to join him in an expedition into theirs.