Albert Barnes Commentary Judges 10:8

"And they vexed and oppressed the children of Israel that year: eighteen years [oppressed they] all the children of Israel that were beyond the Jordan in the land of the Amorites, which is in Gilead." — Judges 10:8 (ASV)

That year - Perhaps the closing year of the oppression, when the Ammonites passed over the Jordan. For it was this crowning oppression which brought the Israelites to repentance (Judges 10:10; Judges 10:15–16), and so prepared the way for the deliverance. Possibly in the original narrative from which this portion of the Book of Judges is compiled, “that year” was defined.

The land of the Amorites - Namely, of Sihon king of the Amorites, (Numbers 21:21; Deuteronomy 1:4; Joshua 13:10; Psalms 135:11).