Causes of World War I
My main source
The history book
One Cause of WWI
Entangling alliances was one reason for the war. In Europe, many countries had defense agreements that later failed on them. If two countries got into a war, then all the other countries were forced to go to war even if they didn't want to.
In the 20th century, countries around the world went into an arms race, which means that everyone wordly was improving their military with more people and weapons.
Imperialism is a nation's conquest of other territories. European countries constantly fought over Africa and Asia for all their natural resources.
Every country wanted to feel dominance over another and this was bad because maybe one country thought they were better than the others and wanted to have a war to see whose country is truly the best.
The assassination of Archduke Franz Ferdinad was one of the main reasons for the war to break out. He was supposed to take over the throne of Austria-Hungary.