There are 4 main causes of WWI:
Militarism-Countries began beefing up their militaries. One country would train some soldiers, which caused nearby countries to get nervous and train their own soldiers, which led to the original country training more soldiers, and on it goes.
Alliances-Because Austria-Hungary declared war on Serbia, Russia, France, and Britain became involved in a matter of days, because they had pre-existing agreements to come to each other's aid.
Imperialism-At the time before WWI, European countries were all scrambling to establish colonies. It was widely accepted that the more colonies a country had, the more rich and powerful they were.
Nationalism-This is the idea that your country is the best, mistakes and faults don't matter. This pride citizens had in their respective countries drove imperialism.
Let me know if you have any other questions!
