In my opinion
In my opinion, the most important long term cause of the war is nationalism. Nationalism is the excessive pride and loyalty in one’s one nation. Nationalism is important because it is the foundation of all the long term causes. This means that you can relate all of the long term causes to it. Firstly, nationalism triggers the need to be the most powerful country which leads to war. For example, in 1861 when Italy became a united country it sparked the growth of nationalism in Europe. This lead to many countries wanting to compete and show that they were the best and most powerful by creating new and advanced weapons (militarism). This shows that when nationalism starts to grow every country thinks that they are the best which prompts them to show off and in order to settle it, war is an option to see who really is the most dominant. Secondly, nationalism leads to disputes between countries which also leads to war. For example, in 1908 the Austria-Hungary empire was losing power so they decided to take over Bosnia to gain more power but Bosnia wanted to join Serbia. This led to the assassination of Franz Ferdinand which was the trigger cause of the war. Also, countries dispute and compete over colonies and the resources that come along with the colonies (imperialism). This shows that disputes fueled by the desire to be the most powerful country lead to war.
Also, Nationalism is the most important long term cause because it relates to alliance systems which also leads to war. Firstly, when countries want to obtain more power they form alliances with other countries. For example, Germany, Italy, and Austria-Hungary formed the Triple Alliance, made up of new countries, Germany and Italy and an old country trying to gain more power, Austria-Hungary. They were against the Triple Entente made up of France, Britain, and Russia. Due to nationalism, these countries formed alliances with other countries to gain more power and resources and then these alliances went to war to compete against each other to see who is the most powerful. Also, because of alliances forming due to nationalism, when one country is getting attacked or threatened both countries alliance members step in to help and defend them which leads to war eventually. For example, when the assassination occurred, Serbia called on Russia because Russia had promised to defend them but then Austria-Hungary called on its ally Germany to help fight. When the other members of the Triple Alliance and Triple Entente heard about this news they decided to jump in and help which led to the war. This implies that when countries form alliances as a result of nationalism and two countries get in a dispute slowly and one by one all the other countries in those alliances get involved and help which leads to one big war. In conclusion, nationalism is the most important long term cause because it leads to alliances being created which fight to compete for power, it leads to countries feeling the need to be the most dominant country, and it causes countries to have disputes over things like land, which all cause war.