The fact that Elvis let himself be drafted into the regular Army I think is just another indicator of his unsophisticated view of the world, and his perception of his role in it. Poor people have often been overtly 'patriotic'. Given his burgeoning career as a singer, he could easily gone into an entertainment troop.

As for his being 'different' after his military stint, I think a lot of that had to do with the sudden death of his mother. She was only in her mid40s when she died; Presley had been extraordinarily close to her.