John has nothing now. Nothing left to do, nobody to be with. Nothing to live for. He has to get his mind off Sherlock because if not, he'll go insane; he'd rather die with dignity, for a worthy cause. Die for, rather than die of. He throws himself back into being a soldier and tries to forget. He works himself sick day after day, giving so much more than he did in his first tour. One day a man is brought to his tent. Clearly not a soldier. Shot in his left side by a man working with a terrorist cell. His face is covered in blood, all features obscured. John begins trying to clean the wound and sew it up, but it's bleeding too much and it keeps soaking through the bandages. The man's skin becomes paler and paler, until it's faded to an ashy grey. The pulse becomes weaker and eventually John loses it. The man's breathing stops. John accepts that he's lost him. He gets a rag and wipes the blood off the man's face.
As he pushes back the wet, messy brown curls from his forehead, his heart stops. Sherlock. Feverishly, he begins trying to revive him. Not because he wants to, but because he has to. After a while, the other doctor in the tent with John pulls him off of Sherlock, saying that he's dead, there's nothing John can do, he did everything he could. John runs out of the tent, yelling Sherlock's name, back into the heat of the battle raging around their little medical tent. John, unarmed, knows full well what he's doing. Is it suicide or murder? Clever little puzzle, Sherlock will love it. Would have loved it. Then he feels it – a sharp pain just below his right collarbone. He falls on the ground, helpless and alone. His partner comes and retrieves him, pulling him back into the tent. He tells him to hang on. Tells him it's alright, he's going to be can't. But John can't. What happens if he lives? He goes back to being alone? No, this is better. He has to do whatever he can to be with Sherlock, even if it means death.
John pushes the doctor away from him and says firmly but quietly, "no." At Mycroft's request, they bury his body with Sherlock's. No casket, just a hole in the ground in a military cemetery in downtown London. One marker, a white cross, but the grave is otherwise unmarked. Whenever people pass by, they'll see the cross and assume it's only for one person, for one life lost. How right they are. Because that's what they were, after all. Two beings that somehow had become completely intertwined into one person, one entity. Sherlockandjohn. They had become identified by what they were to the other. John Watson's best friend. Sherlock Holmes' assistant. One never saw one without the other. They couldn't function apart. Like the hemispheres of a brain, they were brilliant together, practically unstoppable, but without their other half they were useless.
In the end, Sherlock's death had always meant John's as well.