What caused The Great War and how did it become a total war?

Cause:

On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, on 23 July, Austria-Hungary issued an ultimatum to Serbia. Serbia's reply failed to satisfy the Austrians, and the two moved to a war footing.

World War I was a total war. Britain's leaders had extensive powers to reduce threats and at the same time harness its economy through the Defense of the Realm Act. This act was initiated because a shortage of artillery shells in 1915 led to a change in government and new measures were necessary to increase production.