Imperialism: When the nations started to explore different areas around the world, they started to claim them as their territories. This all lead to more competition to see who could gain the most territory. More territory=more power! and if one claim the same territory it could lead to a big war/fight to who will claim it.
But this is my homeland, you can’t just take it!
We will make the best empire!
Nationalism: It is always good to be proud of your country. But these nations proudness and cmmitment to their homelands, lead to an “i am better than you” attitude. Which will make other countries/nations angry saying they are better over the other. That will eventually lead to a war to prove which one is truly the “best”.
We ARE the best!
Your joking, right? We are the best.
No, WE are the best!
The assassination of the Archeduke Franz Ferdinand set the trigger for Europeans to go to war. It lead Austria-Hungary to be angered and with Germany they finally declared war.
I will NOT let you take over my country! We are better!