WWI questions

When and why did WWI break out? and who was on each side?
WWI broke out in 1913, because of the assassination of Austria's Archduke Franz Ferdinand. The war was fought by the Allies (America, UK, France, Russia) and the Central Powers (Germany, Bulgaria, and the Ottoman Empire).
When and why did the US get involved in WWI?
The US got involved in WWI in 1917, after the Germans sank the British ocean liner RMS Lusitania, and the Zimmerman telegram.
What were the casualty rates for the various countries involved?

When and why did the War end?
The war ended in 1918, after the treaty of Versailles was signed.