1. Mutual Defense Alliances
Over time, countries throughout Europe made mutual defense agreements that would pull them into battle. These treaties meant that if one country was attacked, allied countries were bound to defend them. Before World War 1, the following alliances existed:
Russia and Serbia
Germany and Austria-Hungary
France and Russia
Britain and France and Belgium
Japan and Britain
Austria-Hungary declared war on Serbia, Russia got involved to defend Serbia. Germany seeing Russia mobilizing, declared war on Russia. France was then drawn in against Germany and Austria-Hungary. Germany attacked France through Belgium pulling Britain into war. Then Japan entered the war. Later, Italy and the United States would enter on the side of the allies.
Imperialism is when a country increases their power and wealth by bringing additional territories under their control. Before World War I, Africa and parts of Asia were points of contention among the European countries. This was especially true because of the raw materials these areas could provide. The increasing competition and desire for greater empires led to an increase in confrontation that helped push the world into WW I.
As the world entered the 20th Century, an arms race had begun. By 1914, Germany had the greatest increase in military buildup. Great Britain and Germany both greatly increased their navies in this time period. Further, in Germany and Russia particularly, the military establishment began to have a greater influence on public policy. This increase in militarism helped push the countries involved into war.
Much of the origin 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. In this way, nationalism led directly to the War. But in a more general way, the nationalism of the various countries throughout Europe contributed not only to the beginning but the extension of the war in Europe. Each country tried to prove their dominance and power.
5. Immediate Cause
Assassination of Archduke Franz Ferdinand The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria-Hungary. In June 1914, a Serbian-nationalist terrorist group called the Black Hand sent groups to assassinate the Archduke. Their first attempt failed when a driver avoided a grenade thrown at their car. However, later that day a Serbian nationalist named Gavrilo Princip assassinated him and his wife while they were in Sarajevo, Bosnia which was part of Austria-Hungary. This was in protest to Austria-Hungary having control of this region. Serbia wanted to take over Bosnia and Herzegovina. This assassination led to Austria-Hungary declaring war on Serbia. When Russia began to mobilize due to its alliance with Serbia, Germany declared war on Russia. Thus began the expansion of the war to include all those involved in the mutual defense alliances.