World War One may have begun sooner than it did. There had been moments in the Balkans and in Africa regarding Morocco that might have seen war emerge in Europe. However, in such events war was avoided, or perhaps postponed. By 1914, war in Europe was no longer postponed.
In the period before both Germany and France had established alliances in Europe. Germany had the Triple Alliance with Austria-Hungary and Italy. However, France and Britain had increasingly become allies since the Entente Cordiale of 1904. Britain and France expanded the Entente Cordiale to the Triple Entente as Russia also joined the alliance. For the Germans, this was potential encirclement.
Those alliances would come into play when a Serbian nationalist assassinated the Archduke Ferdinand in 1914. Consequently, Austria-Hungary declared war on Serbia.
The emerging conflict in the Balkans sucked both Germany and Russia in. For Germany, as mentioned, had an alliance with Austria-Hungary. As such, Germany offered their fall support to Austria-Hungary in their war with Serbia.
Such a blank cheque could not be ignored by the Russians. Russia had obligations with Serbia. As such, they were ready to provide military support to Serbia to defeat Austria-Hungary.
So it seemed that the Russians and Germans were both on opposite sides in the Balkans. Therefore, war between them began when the Russians mobilized their army in Eastern Europe. Germany duly declared war on Russia.
With Germany and Russia now at war there were implications for Western Europe. The French had their alliance commitments with Russia. However, again it was Germany who declared war on the French in accordance with their military planning. Most likely assuming the French would surely join the Russians at some stage, the Germans wanted to make their own advances first.
The only remaining question surrounded Britain's position. Germany's military planning did not reckon on British involvement, and hopes that they would remain neutral prevailed. The Entente Cordiale was not necessarily enough for them to join the war in support in France. However, Britain's foreign minister supported the French; and the Treaty of London with Belgium was a promise to maintain Belgium neutrality. When the Germans moved their troops through Belgium, Britain joined the side of France and Russia.
As such, World War One began. There were celebratory scenes in Europe as war emerged. Most likely because both sides were expecting a victory, and that the war would last a few months and be over by the end of the year.
Indeed, had the Germany military plan been effective this may have been closer to the mark. The German army invaded France, and the French army duly retreated towards Paris. But the British re-enforced the French lines at the Battle of the Marne. There the Germans could not continue their advance and pulled back.
Germany's plan to defeat France swiftly had fallen apart. The outbreak of WW1 had now begun to turn against the Germans. The war was not over by Christmas and continued to 1918.