For four years, from 1914 to 1918, World War I raged across Europe's western and eastern fronts, after growing tensions and then the assassination of Archduke Franz Ferdinand of Austria ignited the war. Trench warfare and the early use of tanks, submarines and airplanes meant the war’s battles were devastatingly bloody, claiming an estimated 40 million military and civilian casualties, including 20 million deaths. Fighting under brutal conditions, World War I battles on both land and at sea saw mass carnage, but few decisive victories, with some conflicts waging on for months on end. Below is a timeline of the war's most significant battles. Credit for the information History.com