A world war is a military conflict between nations that covers more than one continent. It is typically the deadliest international event in history with an estimated 60 to 80 million people killed.

The causes of world war are complex. Some countries, such as France and Britain, had built up vast empires that drew their neighbours into competition and distrust. Others, like Germany and Russia, were aiming to create their own global empires. This drove them to search for allies and sparked fear of the other empires around them. These structural forces created a dry tinder that was lit by the assassination of Archduke Franz Ferdinand in Sarajevo.

This ignited a global conflict that saw twenty-two countries from six continents declare war in the course of 1914 to 1918. The war became the first truly global war and was fought on multiple fronts across the globe. The major powers were divided into two groups – the Triple Entente of Britain, France and Russia on one side and the Central Powers of Germany, Austria-Hungary and Italy on the other.

The Allied forces eventually grew to include the United States, Canada and Japan. The Central Powers, meanwhile, lost huge areas of territory to the victors and were forced to pay enormous war reparations to the Allied forces. This caused bitterness between the victors and many of the countries they had conquered. The war also led to the establishment of a League of Nations which sought to prevent future world wars.