Many reasons have been given as to the caused of World War I from nationalism to colonial rivalry, and of course we can trace the immediate events that led to the war. After all that is said it must be noted that in many ways it was an accidental war. It was a war that none of those who fought the war actually wanted. The leaders of Europe on both sides of the Alliances were friendly often blood relatives. They led competing countries but many European believed at that time war was unthinkable. Trade between European countries had grown by 1914 to 30% of the average European GNP. The theory that was soon proved to be so wrong was that countries that traded would not fight each other. So why was there war the just the same.
Nationalism had been growing in the countries on Europe, that nationalism was both within the larger powers such as in Germany as well as the small states such as Bosnia which wanted to be free. The nationalism among states resulted more competition for the few remaining colonies and increased military spending. That spending was especially evident in naval spending where the German started building large battleships to compete with the British navy. Naval dominance was the source of British power, especially since it had a small standing army. The main cause of the war however was the series of alliances that had been established between the countries of Europe. Those alliances were defensive in nature, but they obligated the countries to come to the aid of a nation attacked. This situation was aggravated by the military plans of the various countries that forced them to take offensive actions. The most famous of those was the Shefflien plan that the German military operated under. That plan called for the German army to attack France who had an alliance with Russia in case of war with Russia.
Their had been a number of crisis between the powers of Europe in the years leading to the war. The first was the First Moroccan Criss in 1905 in which Germany tried to extend its influence in Morocco but was rebuffed. In 1908 a crisis was created when Austria-Hungary annexed Bosnia which had been part of the Ottoman empire, the Russians objected but in the end gave their acquiescence. In 1911 the second Moroccan crisis occurred when Germany sent a warship to Morocco. They were forced to back down. These events increased the tension in Europe and made it less likely that sides would be willing to compromise in the next crisis.
The immediate chain of events that led to the outbreak of the war was as follows:
June 28th 1914 The Archduke Franz Ferdinand who was the heir to Austrian thrown is shot by a Serb nationalist while visit Sarajevo
July 5th The Germans give the Austrian a green light to to whatever they wish to Serbia
July 23 The Austrian issue an ultimatum to Serbs. Serbia accepts all but one of the points
July 28 Austria rejects Serbian response and declares war
July 29 Austria begins shelling Belgrade
July 30 Russia begin mobilization
July 31 Germans demand that Russia cancels its mobilization
August 1 Germany declares war on Russia,France mobilizes
August 2 Germany demands the right to send troops across Belgium, Belgium Refuses
August 3 Germany declares war on France sends troops into Belgium
August 4 Britain declares war on Germany