World War One occurred due to many causes. The causes could be classified into the underlying and immediate causes.
The underlying causes were: imperialism, militarism, nationalism and alliance system.
Africa and parts of Asia were points of contention among the European countries. They wanted to get more land and raw materials. The increasing competition and desire for greater empires led to an increase in confrontation and this kind of colonial rivalry helped push the world into World War I.
As the world entered the 20th century, an armament race had begun. Britain and Germany were against one another. The German Kaiser ordered the production of new Dreadnought-class battleships. This created more tension between the two nations and helped push the countries involved to war.
In the late 19th century, extreme nationalism was popular among European countries. Different races strived for national glory. It resulted in conflicts when there was a clash of different nationalist movements.
Within Europe, a system of military alliances was established to provide European powers with a sense of security. Two rival alliances were formed. They were the Triple Alliance consisting of Germany, Austria-Hungary and Italy and the Triple Entente consisting of Britain, France and Russia. As a result of Germany’s build-up in naval resources, Great Britain was forced to abandon its isolation policy.
The immediate cause was the assassination of Archduke Franz Ferdinand on 28th June 1914. The assassination occurred during the Archduke's visit to Sarajevo, the capital of Bosnia and Herzegovina.
The German Kaiser, Wilhelm II, provided support through a telegram to Emperor Franz Joseph II. This telegram is known today as the "Blank Cheque." It was this reassurance that prompted Austria to declare war on Serbia, which set off a chain reaction of conflicts. Germany was also held responsible for causing the outbreak of WWI because of this. A harsh ultimatum was issued to Serbia. Austria-Hungary gave Serbia 48 hours to reply. Serbia agreed to all terms of the ultimatum but one.
Outbreak of the War
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.