As a point of reference, here is one of Mauldin's WWII cartoons, so you can see the similarity in style and why I'm sure that one the Hanks letter is by him.
Two movies were based on his "Willie and Joe" characters. The first was Up Front, which was the title of Mauldin's collection of his World War II cartoons. It starred Tom Ewell and David Wayne. And there was a sequel, made the next year in 1951, Back at the Front.
Also, for almost two decades Charles Schultz would pay tribute to Mauldin and have Snoopy head over to Mauldin's house, dressed in army fatigues, to have a couple of beers.