The Peloponnesian War

The Peloponnesian War lasted from 431 to 404 BC. Athens and Sparta were both powerful politically, and each wanted to spread their way of life. Sparta attacked Athens in 431BC, beginning the brutal 27-year-long Peloponnesian War.
 

Sources of Information:

Human Heritage, page 175
http://www.historyforkids.org/learn/greeks/history/peloponnesian.htm

http://en.wikipedia.org/wiki/Peloponnesian_War

http://wsu.edu/~dee/GREECE/PELOWARS.HTM

http://history.boisestate.edu/westciv/peloponn/

http://ablemedia.com/ctcweb/netshots/thucydes.htm

http://www.livius.org/pb-pem/peloponnesian_war/peloponnesian_war.html

http://www.infoplease.com/ce6/history/A0838100.html

http://www.crystalinks.com/peloponnesianwar.html

http://www.richeast.org/htwm/Greeks/war/index.html

http://www.laconia.org/gen_info_literature/Peloponnesian_war.htm

http://www.boisestate.edu/courses/westciv/peloponn/

http://www.mrdowling.com/701-peloponnesian.html

http://www.historyworld.net/wrldhis/PlainTextHistories.asp?historyid=ac46