Completely inaccurate. Truman was in the NG, but was called to active duty (being overage at the time, he DIDN'T HAVE TO GO but did anyway). He commanded a battery in France, seeing heavy combat. The following link provides greater detail. Some people here may not agree with Truman's politics, but he was a patriot and did his duty.
http://www.armyhistoryfnd.org/armyhist/research/detail2.cfm?webpage_id=135&page_type_id=3
Incidentally, Abraham Lincoln was inducted into US Army Ranger Hall of Fame, so apparently whatever he did was enough for the Rangers to claim him as one of their own.
Some may think otherwise here, but it's possible for someone to have political beliefs other than what are popular on FR, and still love and serve this country.
Truman was a cannon cocker, which during WWI meant he seved way behind the lines.
As for Lincoln 6 months of Militia duty doesn't really qualify as anybody could belong to the militia and he never got more than 70 miles from his home in New Salem, and even then his captaincy was political.
Didn't answer the question about the biggest chickenhwak of all though FDR.