Interesting Google results, but you confuse JFK with Papa Joe Kennedy, the Wall Street operator and bootlegger who was ambassador to England. Who cares what a teenaged JFK thought, if he thought anything other that what Papa Joe told him to think. Join the Navy, be a hero, run for President.  
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As for America First being "one of the largest anti-war organizations in American history, there have always been anti-war groups, going back to the Tories during the American Revolution. Lincoln introduced the Spot Resolutions into the US Congress.  Protesting against a war is not the same as being a cheerleader for whichever side is winning at the time.  Bottom line Lindy was an opportunist and a charlatan. 
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Finally when was Lindy disgusted by concentration camps?  At the dinner when he was given the German Eagle?  More likely after the Germans lost the war.