A world war is a huge international conflict that causes a lot of damage and kills a lot of people. The spark that set off the world war of 1914 to 1917 was the assassination of Archduke Franz Ferdinand of Austria by a Serbian nationalist, and rivalry between the countries in Europe over foreign investment, territory and access to markets.
Many of the big European powers were trying to build large empires, which created tension between them. They had fought for their empires in Africa and Asia many times before, and they believed they were superior to the people they were conquering.
The secret alliances that the major European powers had forged and their internal politics led to a state of constant tension. It only took one international incident to trigger a war, and the huge armies that had been built up caused a massive military escalation.
By the end of the year, more and more nations joined the war. The 1919 Treaty of Versailles forced Germany to admit responsibility for the war, give up a lot of its overseas colonies and 13 percent of its European territory and pay reparation (financial damages). This caused great bitterness in Germany, but it allowed it to join the League of Nations that had been set up at the Paris Peace Conference. It was part of President Woodrow Wilson’s “Fourteen Points” aimed at making sure that another world war would never happen again.