The Balkans was an unstable area in the early 20th century. In 1908, the Austrians took Bosnia and Herzegovnia. Russia and Serbia objected, but Germany supported Austria, and so Russia and Serbia backed down. However, Austria and Hungary did not give Serbia its loyalty. The emperor Franz Ferdinand had in mind to give the Serbians their loyalty. However, a military group, the Black Hand, had believed the best way to show the Serbians wanted their independence through fear. By shooting the Archduke Franz Ferdinand on National Day in Bosnia, the Austrians were maddened and sent the Serbians an ultimatum. The Serbians failed to complete the ultimatum. The failure to complete the ultimatum forced the Austrians and Serbians into a war. Then Germany attacked Belgium to attack Russia, and Belgium was considered a neutral country by the other countries. This pushed the British and her allies to attack, causing a World War.
In reality, a war was never wanted- or expected- by the world, so they believed the war was going to be defensive. In fact, the World War was believed only to be a ‘defensive’ war to defend their country. The War ministers also decided that the War would take six months, and that they would be ‘home by Christmas’. Therefore they decided that it would be better to join the War and fight against Germany, and so got involved in the War.
Austria, in the annexation of Bosnia and Herzegovnia, believed that Germany would support Austria if any problems arose. This was owing to the fact that Germany supported Austria in the annexation of Bosnia and Herzegovnia, making Russia and Serbia stop protesting. The Austrians pushed on, knowing- wrongly- that the Germans would back them.
Therefore World War One was begun by an assassination, miscalculations and a misunderstanding of the motifs of the war, and Austria’s beliefs about its allies. This made a war that was against what several war ministers had calculated, and consequently killed 35 million people.