What is the longest period of time the US has not been at war with
someone and what is that date range for this period?

Hello.

The United States' longest period without a war would appear to be the
33-year period between the end of the Civil War (1865) and the
beginning of the Spanish-American War (1898).

Sources:
"America's Wars," hosted by History Channel: 
http://www.historychannel.com/cgi-bin/frameit.cgi?p=http%3A//www.historychannel.com/exhibits/memorial/america.html

"Wars and Conflicts of the United States Navy" hosted by US Navy: 
http://www.history.navy.mil/wars/

Specific start/end dates:

Civil War end:
Lee's Surrender: April 9, 1865
Library of Congress: Civil War Chronology
http://memory.loc.gov/ammem/cwphtml/tl1865.html

Spanish American War
April 19, 1898: Congress Declares War
Library of Congress: Civil War Chronology
http://lcweb.loc.gov/rr/hispanic/1898/chronology.html

search strategy: 
wars, revolutionary, "civil war", spanish-american, korean, "world
war"

I hope this helps.

---

Clarification of Answer by juggler-ga on 05 Apr 2003 21:27 PST

Sorry for that mis-labeled link:

Library of Congress: Spanish-American War Chronology 
http://lcweb.loc.gov/rr/hispanic/1898/chronology.html

During these years [1865-1898 wasn't America fighting the "Indian Wars"?

That's a fair point, Kemlo.
 
If you want to count the battles between the U.S. government and
Indians as "wars," it's probably necessary to say that were no
significant periods without war in the 19th century.
http://www.army.mil/cmh-pg/reference/iwcmp.htm

If you wish to take that approach, then the United States' longest
period without a war would seem to be  the 23-year period between
World War I & World War II (1918-1941).