The problem is that his first real memory is of the Doctor. Later, he understands that he was badly wounded but had somehow managed to survive long enough to be made a prisoner of war, by a secret group that was much, much worse than the Kaiser’s boys.
The Doctor murmurs, I am sorry. I am so, so, sorry.
It’s just before he’s injected by something that ignites like fire through his veins.
Before he blacks out, his last, fading memory was a name.
But he survives.
Everything else, his life, his memory, all of what he must have been before - all of that perishes instead.
The Doctor tells him, grief and guilt and horror written all over his face, Remember. Remember this much. You are a good soldier. You are a good man. You must live.
Whatever else has been burned out of him, the Soldier at least remembers how to think, how to plan, how to escape.
Eventually, he manages to get out. He takes the Doctor with him.
This is what the Soldier doesn’t know. Elated by the success of the Doctor’s Super Soldier Serum, the man known as Johann Schmidt will inject himself with it. The results are far more horrifying. But he is a practical man and a few….cosmetic changes in his appearance can be easily remedied. He learns to embracehis change, so much so that he wears the moniker of Red Skull with pride.
The Soldier, the first true success of Dr. Abraham Erskine, disappears in the red tape and bureaucracy of an army too busy trying to clean up what was left behind of the first World War. A soldier with no memory of his name and his family can be easily overlooked. And the Doctor is far too keenly aware that this man has already suffered much - so he keeps his silence on what has been done to him.
The Soldier selects a new name. John Smith is common enough. It will do.
He tries not to think of the woman he can only remember as Sarah too much. He tries not to think about the child sleeping beneath her heart when he’d left for the front, a child he would never, ever know. He is a Soldier of Fortune now. The Doctor has given him something that he would like to think of as his standing orders, now that he has nothing else.