“Israel took away my land.” This is revisionist history. Israel did no such thing. At the time of the Exodus, the Amorites controlled the land that Israel conquered, as Jephthah pointed out to the king of Ammon (Judg. 11:15-26). The record of Israel’s conquest of the Amorite territory east of the Jordan River is in Numbers 21:21-35 and Deuteronomy 2:24-36; 3:1-11. God specifically told Israel to not invade the Ammonite territory (Num. 21:24; Deut. 2:19, 27). The king of Ammon defined the land that Israel supposedly took by three rivers, the Arnon to the south, the Jabbok in the north, and the Jordan River on the west.
Often people lie when they want something, and the wise person is aware of that. Sadly, some people are so convinced of their lie (or greedy for what they want) that it leads to conflict, and the godly person must be prepared to enter into that conflict to protect his name, rights, and property. Evil will enlarge its boundaries unless people stand against it.
“when he came up out of Egypt.” The “he” represents Israel as a corporate entity, a singular. The text does not read “they.” Also, it is noteworthy that the king of Ammon acknowledges that Israel came up out of Egypt, something that many historians do not want to acknowledge today.