War against the Nazi's wouldn't have had to happen if we hadn't bankrupted Germany after WWI and left it vulnerable to the kind of craziness that Hilter and his bunch pushed. Yes, they did come to power and violence did stop them, but it didn't have to be that way. What I mean to say is that we had a lot of opportunities to keep something crazy from happening and we missed them all and left ourselves with only the option of war. Fighting them was our worst, last option because the world was too stupid and selfish to look to the future.
We can make violence unnecessary if we plan properly and do whatever we can to educate and help people. If we do that and just make the world a better place for everyone you'll only see the kind of person-to-person violence from mentally disturbed people that can be solved with non-lethal means most of the time. I don't believe that there is ever not a non-violent option to solve a problem, just that we often aren't careful enough, smart enough, or willing enough to choose the non-violent option.