This is a good question because I think most readers would defend the notion that the story's central conflict is an internal conflict. Lyman and Henry both struggle internally. Lyman struggles to come to terms with the massive changes that his brother has gone through due to seeing combat. Henry's internal struggle is with returning to "normal life," and that is what Lyman struggles to understand. With that said, there are some external conflicts present in the story. There is a bit of man vs. man conflict that exists between Henry and Lyman. Lyman wants his brother to do the same kind of stuff that they used to do, and Henry just isn't willing or capable of it anymore. A good example of a small conflict between them is the car itself. They get into a small argument about its condition and what should be done about it.

We start talking. He said he knew what I'd done with the car. It was obvious it had been whacked out of shape and not just neglected. He said he wanted to give the car to me for good now, it was no use. He said he'd fixed it just to give it back and I should take it.

"No way," I says, "I don't want it."

You might be able to make a case for a man vs. society external conflict. Henry returns a very different person, and much of his family and friends don't understand the changes. They have hopes and expectations for Henry that he simply can't meet.