Leftists, pacifists, and anti-war demonstrators think they are doing the world good by keeping their country from engaging in war. In actual fact, American pacifists in the 1930s made possible the appeasement of Hitler which led to World War II. Let me explain.
After World War I, many Americans came to view our intervention in that war and the subsequent peace settlements as a tragic mistake. Franklin Delano Roosevelt, on the other hand, had read Mein Kampf and realized that Hitler was a madman and was a threat not only to Europe but to America as well (1). Unfortunately, because of the neutrality acts Roosevelt was prevented from directly helping any country fight against Hitler.
Worse, when Roosevelt tried to persuade Britain to take a strong stand against Hitler, they were persuaded to do otherwise because they realized that because of the pacifists no help would come from America: From the book "No More Killing Fields" by David A. Hamburg:
[Chamberlain:] ... the Americans were not a useful counterweight to Germany: "It is always best and safest to count on nothing from the Americans but words. ...
Fearing Britain would have to face Germany without America, Chamberlain took the path of appeasement, which only encouraged Hitler to go further.
Because of their fear of war, because of their cowardice, American pacifists encouraged war where a more aggressive stance might have stopped Hitler altogether. Tens of millions dead; this is what pacifism wrought.