John James Rambo was not the smartest of people. He wasn't good for sitting behind a desk and working, he was too active for that and he couldn't have the deep or philosophical discussions, he was to violent for that. This didn't mean he wasn't smart, quite the opposite in fact he was very intelligent, especially on the battlefield. So dangerous was his cunning and skill that he was a legend amongst the US special forces.
All of this was fine, except for one, crucial fact, he was alone, so very alone, he was tired of fighting, and war and death. He was planning to head back to Asia after his adventure in Afghanistan but he decided to take a quick detour to England to try and reconnect with some family from his mother. He wasn't expecting to stay, but he figured it was time too do something good. He planned to pass on his experience to someone, he didn't know who but he would when he saw them.
In the end it was all for nothing. He lamented as he sat in the corner of the living room of 4 Privet Drive. All of his mother's relatives were fat and lazy. So caught up in their day to day lives that they would disregard his gift as amusing stories.
Just as he was about to get up and leave he felt his hair stand up and had already turned his head before he remembered to restrain himself. There, staring curiously next to him was a boy, small, dark hair, green eyes, very thin and practically swimming in his clothes. Overall the kid was very much a non threat, but in his eyes, John saw something he had only seen in the mirror. John didn't know how he knew, but he knew, that this kid would need his help more than he could fathom.
John wasn't one for mentoring, let alone raising kids, which was why after looking around and seeing that the Dursleys were to focused on the large, spoiled, child to pay John any mind, he quickly reached into his bag and pulled out a large package wrapped in brown paper. He handed it to the boy, who took it with a look of bewilderment. When the boy looked back up at him, John only put a finger to his mouth in a gesture for silence. After that John Rambo got up with his things, and left without another word. The Dursleys never noticed him leave, nor did they notice an eight year old Harry Potter quietly slip into his cupboard under the stairs to open and subsequently hide his unexpected gift.
Three and a half years later, Harry returned from his first year at Hogwarts. Sitting on his bed he skimmed the journals that John had given him, detailing John's time in the military and his adventures since. Half way through, an epiphany hit him. He realized that eventually, Voldemort would find a way to bring himself back to life one way or another. When that time came, Harry, The-boy-who-lived, the one that got away, the one who supposedly killed Voldemort in the first place would be at the Top of Voldemort's hit list.
When that epiphany hit him, Harry first felt a sense of dread. What could he, a kid do against a fully grown Dark Lord who's very mention made the Wizarding world tremble with fear. Then, that fire that only John Rambo would recognize burned bright as Harry Potter decided in his small bedroom that he would be ready. He would face Voldemort when he came and make the dark tosser rue the day he messed with him.