The children of Israel again did that which was evil in the sight of Yahweh: and Yahweh strengthened Eglon the king of Moab against Israel, because they had done that which was evil in the sight of Yahweh.
The Moabites and the Ammonites originated in the incestuous union of Lot and his daughters  (Genesis 19), but this physical kinship with the Jews did not mitigate their unwavering enmity against Israel.  Although Moab was the chief predator in this oppression of Israel, it is not surprising that both Amalek and Ammon were Moabite allies.