The Israelite people were commanded by God to avoid any conflict with the people of Ammon during their march to the Promised Land. When Israel had come into their land, in the days of Jephthah, the Ammonites oppressed the Israelites who settled east of the Jordan while claiming it was their ancestoral land when Israel took it; in fact it belonged to the Amorites when Israel took the land. Jephthah crossed over to the Ammon and faught against them, dubduing them before Israel.
Very early in the reign of king Saul, Nahash the Ammonite besieged Jabesh-gilead. The people of the town offered a treaty of peace and agreed to serve Ammon. Nahash would only accept the agreement if he was allowed to gouge out the right eye of all the people. When Saul heard of this he a yoke of oxen, cut them into pieces, and sent them throughout Israel saying that whomever does not come out to battle Ammon will end up like these oxen. The fear of the Lord fell upon the people and they all came out for battle. Saul and the people struck down the Ammonites from morning until the heat of the day. Those who survived were scattered.
During the reign of king David, Nahash died and his son Hanun took the throne. Upon hearing of this David sent a message to the new king in hopes that he could have an agreeable relationship with Hanun as he did with his father. Hunan's advisers convinced him that the messengers of David were sent not to comfort but to spy out the land in order to overthrow it. So Hanun shaved off half the beard of each, cut off their garments in the middle of their buttocks, and sent them back to Israel. Hanun realized he had angered David and hired the Syrians of Beth-rehob and Zobah, the king of Maacah, and the men of Tob to fight with him against Israel. Upon hearing of this David sent Joab and the army of Israel to Ammon where they defeated the armies and subjugated the Ammonites. When David captured the royal city of Rabbah he took the crown from Hanun's head and placed it on his own and returned to Israel with the spoils of war.
After the reign of king David, the Ammonite hostility towards Israel is evident throughout the rest of the Bible. They joined the Moabites and some Meunites in their attack of Judah during the reign of king Jehoshaphat, joined the Chaldeans against Judah, the murder of Gedaliah of Judah, and the opposition to Nehemiah during the rebuilding of the walls of Jerusalem.