Ninja were mostly made of spies. Assassination was not the main focus and there are few documented accounts of such. Some ninja were ex samurai like Hattori Hanzo.
One could say they were the enemy of the samurai. The Samurai were like European Knights per se. They swore loyalty to their lord, the emperor, and Japan.
They were a ruling class, a few born to nobility. Professional soldiers sworn to follow the code of bushido, but few followed that code in times of war where rape and pillaging was common. That's a short answer.
For more info simply look it up.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.