Question

what devastating historic event compelled many European countries to work together in order to prevent future conflicts?

Answer 1

The correct answer is World War II

Following World War II, the United Nations were found with a goal of preventing such monstrosities in the future.

Answer 2

There were several events in history that can be considered devastating, especially in the European countries. What time-frame are you talking about? 

I want to say WW2 because of the Holocaust.