In opinion I think the most significant cause was system of Alliances because if there wasn’t any alliance there would be no war. Alliances were agreements or promises between two or more countries that allow them to support and help another. If other countries didn’t start coming into Austria-Hungary’s war with Serbia, it would not have turned into a major war. This was the cause that involved may other countries into the war. The countries involved in World War One were Austria–Hungary, Germany, Britain, Italy, Russia and France. When Archduke Franz Ferdinard and his wife got shot by Gavrilo Princip, in revenge Austria-Hungary, backed by Germany, declared war on Serbia. Russia, backed by France, supported Serbia. Then Germany invaded Belgium and attacked France, drawing Britain into the battle.
The third cause of the war was Imperialism. The European countries wanted to conquer other lands and to build an empire to show that they were the toughest. France lost their most wealthy lands. All the great powers were competing for colonies and land. The British feared Germany in Africa. Great Britain had a huge empire, the biggest in the world at the time, and Germany didn’t, they wanted a piece of the action and they teamed up with Alsace and Lorraine to form a new bigger Germany. Who went on to have the Arms race with Britain. Great Britain had great control over the sea as they had loads of the ports around France and the south of England, however if they lost their ports they would lose their trading empire with India and other countries. France and Germany caused damage. Imperialism was a long term cause in World War One.
Another factor for the war was Nationalism. On 28 June 1914 the spark came. The nationalist Serbian terrorist group called ‘Black Hand’ killed the arch duke Franz Ferdinard. Archduke Franz Ferdinard and his wife Sophie made a tour of Bosnia. As they drove through the streets of Sarajevo, a Serbian killer shot them both dead. In revenge Austria-Hungary, backed by Germany, declared war on Serbia. Russia, backed by France, supported Serbia. Then Germany invaded neutral Belgium and attacked France, drawing Britain into the battle. Nationalism was one of the long-term causes. Nationalist were people who were fiercely delighted of their country, ready to defend their nation and who would take any action. The alliance systems made countries feel more protected. The Serbians and the Bosnians wanted to become one country due to their similarities and they agreed to assist one another in the event of war.
In 1882, Germany, Italy and Austria-Hungary formed an alliance, called the Triple Alliance, capable to protect each other in the World War One. In 1904, Britain joined France in the similar alliance. They were joined by Russia in 1907 to form the Triple Entente. Nationalism was an important cause.
In conclusion, the long term causes were militarism, imperialism, alliance and nationalism. The short term cause was when the Archduke Franz Ferdinard and Sophie were shot by Gavrilo Princip and Austria-Hungary declared war on Serbia. I have given several of reasons above why WW1 broke out in 1914. The most important four reasons that war broke out in 1914 are underlined.