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.
That's just flat out wrong. Did you read the link provided? Truman was under fire numerous times.
I'll take the word of the Rangers re Lincoln.
As for FDR, he was 37 years old at the entry of the US into World War I.