WWI

When and why did WWI begin, and who were the combats?
 "Triple Ententeof the United Kingdom, France and the Russian Empire) and the Central Powers of Germany and Austria-Hungary. Although Italy had also been a member of the Triple Alliance alongside Germany and Austria-Hungary, it did not join the Central Powers, as Austria-Hungary had taken the offensive against the terms of the alliance.^[11] These alliances were both reorganised and expanded as more nations entered the war: Italy, Japan and theUnited States joined the Allies, and the Ottoman Empire and Bulgaria the Central Powers." The war started because of the assassination of Arch Duke Franz Ferdinand from wikipedia

When and why did the US get involved in WWI?
The US got involved on April 6, 1917. The US entered the war because of unrestricted submarine warfare by germany.  history learning site

What were the casualty rates for the various countries involved?

http://www.worldwar1.com/stats/pmisspow.gifgraph 

When and why did WWI end?
it ended November 11 1918 and ended because of the Treaty of Versailles