Because you are using the sorrow of war to justify the means of war. It's a cyclic argument with each new participant upping the stakes; each time bringing about more suffering.
In the case of the America vs Japan conflict during WW2, America's "humanitarianism" lead to two nuclear bomb being dropped causing around two hundred thousands casualties - many of who suffered long deaths from radiation exposure. Yes it shortened the war, but at what cost? We'll never know which hypothetical scenario would have saved the most lives but it's fair to say that America's involvement did contribute to large scale suffering too.