The World War I was a war that originated in Europe and got over 70 million combatants involved in it. It began on July 28, 1914 and lasted till November 11, 1918. The following causes of the WWI could be indicated: alliances, imperialism, militarism, nationalism, and Moroccan and Bosnian crises. Within the pre-war period several alliances were signed, and the members of those aligns were obliged to declare war on the countries which attacked their allies.
Those alliances were the 1879 Dual alliance of Germany and Austria-Hungary against Russia, the 1881 Austro-Serbian alliance to stop Russian gain of control over Serbia, the 1882 Triple alliance to prevent Italy from aligning with Russia instead of Germany and Austria-Hungary, the 1894 Franco-Russian alliance, the 1904 Entente Cordiale (agreement between Britain and France), the 1907 Anglo-Russian Entente, the 1907 Triple Entente (counteraction of Russian, Britain and France to Germany), and the 1914 Triple Entente between Russia, Britain, and France not to sign separately for peace.
Imperialism ensured rise of rivalry between Germany and Britain and France for their colonies. Armies of major competitors have doubled in the 1870-1914 period. Besides, the German and Italian nationalists wanted their countries to be free and unified again. The formal cause was the assassination of Franz Ferdinand, the Archduke of Austria, by a nationalist supporting Bosnian Serb. All previous alliances resulted in the involvement of Germany, Russia, France and Britain. Regardless of the 1882 Triple alliance, Italy did not participate in attacks on other countries until 1915, considering that alliance to be primarily defensive.
On July 28, 1914, after declaration of war on Serbia, Russia stepped in for its protection, while Germany stepped in to assist Austria-Hungary. On August 1, 1914, Germany declared war on Russia. France stepped in to assist Russia, so Germany declared war on France. Germany invaded Belgium for better position before attacking France (Schlieffen plan). On August 4, 1914, Britain declared war on Germany. It was a 2-front war: Western (France-Germany) and Eastern (Russia-Germany). Mobilization of troops and workforce was conducted for efficient use of available resources. USA remained neutral until the sinking of the Lusitania cruise liner on May 7, 1915. On April 4, 1918 America entered the WWI. The WWI could be avoided if Serbia had agreed to all conditions of Austria-Hungary instead of their partial acceptance.