What was a war fought by the US in the 1900's?

The Civil War!

[EDIT] No! The Civil War was fought in the U.S. from 1861-1865.

There were no U.S. wars fought in the decade of 1900-1909. The Spanish American War ended right before this decade began and World War One began in Europe in 1914. (We did not join World War One until 1918.) However, in this decade Russia and Japan were at war from 1904-1905.