After a period of nomadic existence, the Ammonites settled north of the Moabite kingdom in the 13th century BCE and founded their capital city Rabbath Ammon (present day Amman). Like their neighbors to the south, the Ammonites often attacked the newly settled Jewish empire in Canaan. Similarly, after the destruction of the First Temple, Ammonite king Baalis instigated the murder of Gedaliah, ensuring the downfall of the Jewish nation. The Ammonites were also involved in the Hasmonean conflict, in which Judah the Maccabee defeated Syrian and Ammonite forces.
Sources: Bridger, David. Ed. The New Jewish Encyclopedia. NY: Behrman House, Inc. 1976.
Schreiber, Mordecai (ed.). The Shengold Jewish Encyclopedia. Shengold Books. 1998.
Telushkin, Joseph. Jewish Literacy. William Morrow and Company, Inc. 1991.