World War I was started through many different circumstances. The main people who started the War were the Slavs.
There were the East Slavs, the Russians and the Ukrainians. There were the West Slavs and, mainly, there were the South Slavs. These last consisted of the Serbs, Bosnians, Croats and Slovenes. All the countries, namely Serbia, Bosnia, Croatia and Slovenia, were in the Balkans, an area within reach of the Austrian-Hungarian Empire.
The pan-slavism movement was made by the Slavic Russians to free the non-Slavic Russians from foreign rule.
Germany had, in the Franco-Prussian War of 1870, taken from France the territory of Alsace-Lorraine, which the French bitterly regretted. For this reason France went against the Germans in whatever they did. Germany’s Prime Minister, Otto van Bismarck, wanted to stall any possible alliance between the already powerful France and Russia. If that happened Germany would be fenced in on all sides, leaving no escape route.
So Bismarck created an alliance with Russia against France. But a while later, Emperor William II of Germany dismissed Bismarck and failed to keep the alliance with Russia.
Before being fired Bismarck also cemented the Triple Alliance, which consisted of Germany, Austria-Hungary and Italy.
Then, in 1894, what Bismarck had feared happened. The French formed an alliance with Russia, and Britain joined ten years later, forming the Triple Entente.
The Bosnian crisis happened when Austria-Hungary, which was supposed to be protecting Serbia and Bosnia, annexed these territories for itself. Outraged, the Serbs, who had wanted to become an independent country, appealed to Russia for help. Russia responded, but backed away when threatened by Germany. This led to a later event, the assassination of Archduke Francis Ferdinand.
The First Balkan war happened between Greece, Serbia, Bulgaria, Monte Negro and the Ottoman Empire. All of the above listed were engaged against the Turks. The end result: territory for the Balkans.
The Second war this: Serbia, Romania, Greece and the Ottomans fighting against Bulgaria over the territory gained in the First Balkan war. Eventually Bulgaria was defeated.
Meanwhile, Germany was building a navy. They wanted a large one, capable of matching up to any other. Britain was also fabricating a navy, one that had to be larger than any other European one, because they had to maintain control over the Channel in order to feed themselves.
Then came the final blow: Archduke Francis Ferdinand and his wife Sophia were murdered by a Serbian terrorist of the Black Hand terrorist movement. He was killed because he had proposed that the Serbs and Bosnians become independent countries under the rule of the Austrian-Hungarian Empire. He had wanted to create a triple monarchy. For this he was murdered. This acted as a trigger for the war as, one after another, the countries around Europe responded.
This is the coming of World War I.