Sanballat was a major opponent towards Nehemiah while the walls of Jerusalem were built and reforms were implemented towards Jews after their return from Babylonian exile. Sanballat was a prominent Samartian, a term used for non Israelite who lived in Centeral Israel. Sanballat, as per Sayce is related with the Assyrian "Sinballidh," and means "Sin has vivified." Moreover, Sanballat was also known as "the Horonite," and was connected with Tobiah the Ammonite and Geshem the Arabian. However, a Persian, his home was obviously at Samaria, where the term Horon possibly originated from Bethoron.

Once Sanballat saw the returning Jews commencement to rebuild the utterly devastated city of Jerusalem, he mocked them and attempted to discourage them to the point that they would give up.(1) In spite of Sanballat's resistance, the reconstruction was a success.(2) Jews heavily armed themselves while they worked in order to protect themselves against an attack by Sanballat.(3) Attempts by Sanballat to assassinate Nehimiah were fruitless.(4) Sanbalat's daughter became the wife of one of the sons of Judah, a grandson of the high priest, Eliashib, much to the grief of Nehemiah. The high priest was driven out of Jerusalem on the ground that he had defiled the priesthood.(5)

References:

1) Nehemiah 4:1-2
2) Nehemiah 4:7-8
3) Nehemiah 4:16-18
4) Nehemiah 6:1-12
5) Nehemiah 13: 26-29
http://jewishencyclopedia.com/view.jsp?artid=218&letter=S
http://www.keyway.ca/htm2003/20031206.htm
http://www.mun.ca/rels/restmov/texts/schambers/ww/SVN.HTM