What Happened to American Optimism?

   In "The Christian Faith and the World Crisis," Niehbur writes, "We are well aware of the sings of all the nations, including our own, which have contributed to the chaos of our era." Harry Emerson Fosdick echoes the same sentiment when he says "the bill of particulars has often been written, and all but unanimous judgment seems to be that we, the democracies, are just as responsible for the rise of the dictators as the dictatorships themselves, and perhaps more so." 

     Of course, Fosdick and Niehbur reach completely different conclusions when it comes to American foreign action, but both hold a fundamental pessimism about the effectiveness of our nation and modern war that we did not see just two decades previous in the American attitudes toward the Great War.  Were any optimists left, or were all sides of an argument approached from this new understanding of war?  What caused this pessimism about war? Was it the destruction caused by WW1, despite the relatively light impact it had on the United States specifically? Was it the failure of the Great War to literally end all war? Or was it caused by other factors, like global depression?  Was the pessimism an American idea, or more of an import from Europe?