On June 24, 1914 Archduke Franz Ferdinand and his wife were assassinated by terrorists called "Black Hand." All nations we're already ready for war, but their breaking point was the assassination. It was as if everyone was done with eachother's nationalism and wanting to be better than everyone else, and since the assassination was a terrorist attack strengthened the Serbian nationalism.
The Franco-Prussian War was between France and Prussia, in 1870 and 1871. Chancellor Otto Bismark planned this whole thing to bring together a more unified Germany. He spreaded rumours and instagated the war. France and Prussia declared war, and Germany was allied with Prussia. Prussia and Germany won and France forever hated the Germans and wanted revenge.
The Pan-Slavic Movement was caused because Slavs needed to get to together, no matter borders of language and geography, and be independent and have unity. This movement was cultural for the Slavs, and political for Russia and Balkan. Throughout the war, Russia wanted to protect the Slavic people in Serbia.
Serbia encouraged terrorists groups, as nationalists. "The Blank Hand" was only one of probably many terrorism groups to destroy Europe. One of their most famous attacks was the assassination of the Archduke. Their secret societies wanted to promote the liberation of Serbs.
The Russian Revolution broke out because the peasants and working class of Russia revolted against Germany. The Bolsheviks were the group of revolutionaries who led this. The Russians wanted to stay united and independent, being ran by themselves and nobody else.
Germany had became a very large nation after a very long time. Their military grew humungous, their nationalism was extrordinary. They were intimidating by all their neighboring nations. France still wanted to get revenge and take them down, even though they were very large. This led to the war because Germany was ready for whatever came to them.