World War I arguably began in Sarajevo, as the proximate cause of the conflict was the assassination of the Austrian Archduke Franz Ferdinand by a Serbian nationalist. The killing exacerbated the existing tensions between Austria and Serbia, leading to the first declaration of war.Know More
Serbia was under the protection of Russia, which was in a military alliance with Great Britain and France. Austria's declaration of war against Serbia motivated a Russian counter-declaration of war against Austria. Germany then declared war on Russia in support of Austria. The German declaration triggered British and French declarations, and most of Europe's armies mobilized for war.