The Pawnee were often at war with other tribes. The Cheyenne and Sioux were their main enemies. They planned surprise attacks, called raids. Also, seven months a year they went out to hunt buffalo. The sick, old, and very young were left behind. Because this was a major weakness for the Pawnee, the Cheyenne and Sioux attacked them.
Bibliography
http://college.hmco.com/history/readerscomp/naind/html/na_028300_pawnee
http://crystalinks.com/pawnee.html
http://www.olden-times.com/OldtimeNebraska/n-csnyder/nbstory/story28.html
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http://en.wikipedia.org/wiki/Pawnee#Religion
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"Pawnee." Encyclopedia Americana. 2001
Lacey, Theresa Jense. The Pawnee. United States of America: Chelsea House Publishers, 1996.
Fradin, Dennis B. The Pawnee. Chicago: Children's Press, 1988.