Fighting took place mainly in Europe and the Middle East , as well as in parts of Africa and the Asia-Pacific , and in Europe was characterised by trench warfare ; the widespread use of artillery , machine guns, and chemical weapons gas ; and the introductions of tanks and aircraft.
World War I was one of the deadliest conflicts in history , resulting in an estimated 10 million military dead and more than 20 million wounded , plus some 10 million civilian dead from causes including genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic. The causes of World War I included the rise of Germany and decline of the Ottoman Empire , which disturbed the long-standing balance of power in Europe, as well as economic competition between nations triggered by industrialisation and imperialism.
Growing tensions between the great powers and in the Balkans reached a breaking point on 28 June , when a Bosnian Serb named Gavrilo Princip assassinated Archduke Franz Ferdinand , heir to the Austro-Hungarian throne. Austria-Hungary held Serbia responsible, and declared war on 28 July. After Russia mobilised in Serbia's defence, Germany declared war on Russia and France , who had an alliance.
Germany's strategy in was to quickly defeat France, then to transfer its forces to the east, but its advance was halted in September , and by the end of the year the Western Front consisted of a continuous line of trenches stretching from the English Channel to Switzerland.
The Eastern Front was more dynamic, but neither side gained a decisive advantage, despite costly offensives. Italy , Bulgaria , Romania , Greece and others joined in from onward. Major battles, including at Verdun , the Somme , and Passchendaele , failed to break the stalemate on the Western Front.
