Answer:
There were multiple causes
Step-by-step explanation:
I'm assuming your question is referring to World War 1, but here's my explanation. Before World War one, there was Napoléon and if you don't know, well then he screwed the whole of Europe for a while. Following his defeat, there was the congress of Vienna in which all the great powers of Europe (Russia, France, Austrian Empire (later on Austria-Hungary), Prussia (later on the German empire), and Great Britain) decided to discuss on how they can bring a balance of power to Europe. The thing about the congress of Vienna was that it failed to stop Prussian power from rising. I would like to tell you about all the battles that happened to lead to all these alliances being here, but I'm afraid I'll make my explanation to lengthy. Here's my grossly simplified version, Prussia unified most of Germany which led to a battle against France in which they won (Franco-Prussian war); Russia wanted to have control over the Ottoman controlled Dardanelles and France and Great Britain said this was a big no no and so this led to a war with Russia going against the Ottoman empire, France, and Great Britain (Crimean war), and the Russians lost, but the thing though was that Russia asked Austria-Hungary for help but they said no even though Russia helped Austria-Hungary against its war with Prussia, and then there was a whole naval arms race with Germany trying to build a bigger navy than the British, but Britain was able to keep their naval dominance under control. At this point you could say that Germany hates France and UK, then Austria Hungary hates Russia, and they they both were German speaking empires so they both decided to make an alliance which was called the central powers. And because of this, France, UK, and Russia decided to create the Entente to go against the the central powers. Kicking on one of the boldest wars in World History.