Human's capability to turn into violence for issues that can be solved diplomatically.
Explanation:
For the people who live in that period, the idea of World War I probably sound ridiculous. People thought that they're already passed the age of violence and murder. But they couldn't be more wrong.
World war I might be triggered by the Murder of Archduke Ferdinand of Austria. But the fact that so many countries participated in it indicates that it actually the result of years of political problems, competition to obtain power/influence, and nationalistic pride.
If the countries at the time decided to set aside their egoistical nature and choose diplomatic approach to solve their problems, They could've prevented more than 20 million deaths.
