Countries through Europe made defense agreements between each-other that would pull them into combat. If one country was attacked, the allied countries were certain to defend them. Before WW I, the following alliances existed: Russia and Serbia, Germany and Austria-Hungary, France and Russia, Britain and France and Belgium, Japan and Britain.
Imperialism is when a country increases their power and wealth by bringing other territories under their command. Before WW I, Africa and parts of Asia were points of disagreements amongst the European nations. This was true because of the raw materials these areas could provide. The increasing competition and desire to gain land and wealth by capturing land.
As the world come to the 20th century, a weapons race had begun. By 1914, Germany had the highest increase in military. Great Britain and Germany both significantly increased their navies in this time period. In Germany and Russia particularly, the military began to have a greater influence on public policy. The increase in militarism helped push the countries that later involved to war.
Much of the cause of the war was based on the desire of the Slavic peoples in Bosnia and Herzegovina to no longer be part of Austria Hungary but instead be part of Serbia. Nationalism led directly to the War. But in a more general way, the nationalism of the different countries throughout Europe added not only to the beginning but the…