Perhaps he felt the need to make a living for his children...perhaps it was a selfish decision, based on his career interests...but it is all speculation beyond the fact that he didn't serve, since he didn't like to talk about it, and now that he is dead we can't ask. He did serve America, both at the time by making movies that entertained and inspired other Americans...and he became a great spokesman for America as a patriot.
He didn't refuse to serve, as did some...and he obviously later wished he had served. Many of us have made changes in life, and the persons we are now are not what we once were. Nonetheless, I like the man as a actor, as a person, and as an American. He wasn't perfect, and neither is anyone else...but I'd much rather choose his company than someone like Sean Penn, Jane Fonda, or Michael Moore.