WWI started in 1914, and ended in 1918. To this day there are many theories that are written, and unknown about the causes of this WWI. Some of them include, imperialism, and alliances. Imperialism added to the causes of the first world war because, Great Britain, Germany and France needed foreign markets after the increase in manufacturing caused by the Industrial Revolution. These countries competed for economic expansion in Africa. Although Britain and France resolved their differences in Africa, several crises foreshadowing the war involved the clash of Germany against Britain and France.
Alliances were a major cause of WWI. They (alliances) made it so that if two powers in went to war, all the powers would go to war. Like, when Russia moved to threaten Austria-Hungary into leaving Serbia. This alerted the Germans who were Austria-Hungary's enemies, that war was likely.
This would mean that Germany would have to fight Russia. An attack on Russia would bring France and Britain into the war. Germany would also then have to face war on two fronts, (France and Russia). To them it made sense to attack France first and thus defeat them, and then deal with Russia. Although all the causes of war have probably never been discovered, and never will be clear, there are however many more than what is listed here. But, for our sake, historians have done what they could to research, and provide our generation today, with information that is likely to help us understand what the Great War that started in 1914, and ended in 1918 was all about..