-Growth of ethnic nationalism throughout Europe and Middle East
The idea of the need of a "pure race" or of races of usurpers in the countries were getting common.
-Colonial groups looking for independence from imperialistic powers
This alone was more than reason for many to attack others from their own country. The problem is that the line is always blurry when it comes to decide the targets of the attack and who are actually the supporters of the enemy that deserve to be attacked. In the end, many just decided that just being against the cause was enough to add someone to the casualties.
-Spreading of communism and socialist ideas throughout the world
This enough generated many problems with the war between classes and the very troubling ideas of non existence of private properties and many other things that led straight up to physical conflicts, abuse of power and so on.
