The children of Ammon made war - They had invaded the land of Israel, and were now encamped in Gilead. See Judges 10:17.

And it came to pass in process of time,.... Some time after Jephthah had been expelled from his father's house, and he was become famous for his martial genius, and military exploits; or at the close of the eighteen years' oppression of the children of Israel by the Ammonites, or some few days after the children of Israel were gathered together at Mizpeh, that the people and princes of Gilead were preparing for war with Ammon, and were thinking of a proper person to be their general:
that the children of Ammon made war against Israel; not only passed over Jordan again, and encamped in Gilead, but began to attack them in some place or another, at least threatened them with it, and made motions towards it.

THE GILEADITES COVENANT WITH JEPHTHAH. (Judges 11:4-11)
in process of time--on the return of the season.
the children of Ammon made war against Israel--Having prepared the way by the introduction of Jephthah, the sacred historian here resumes the thread of his narrative from Judges 10:17. The Ammonites seem to have invaded the country, and active hostilities were inevitable.

But when the Ammonites made war upon Israel some time afterwards, the elders of Gilead (= "the princes of Gilead," Judges 10:18) went to fetch Jephthah out of the land of Tob, to make this brave warrior their leader. In Judges 11:4 the account of the war between the Ammonites and Israel, which is mentioned in Judges 10:17, is resumed, and its progress under Jephthah is then more fully described. "In process of time" (מיּמים, a diebus, i.e., after the lapse of a long period, which cannot be more precisely defined), sc., after the expulsion of Jephthah from his home (see Judges 14:8; Judges 15:1; Joshua 23:1). קצין signifies a leader in war (Joshua 10:24), and is therefore distinguished in Judges 11:11 from ראשׁ, a chief in peace and war.

Made war - The Ammonites had vexed and oppressed them eighteen years, and now the Israelites begin to make opposition, they commence a war against them.