I'm trying to identify a short-story, definitely written before the 1980's.
The story was told in four parts;
1. There was a biography-like introduction which lays out the story; the bizarrely meteoric rise of a private soldier to President of Earth and his similarly mysterious assassination.
2. An alien consciousness is drifting around, flicking through various universes. It passes a universe of matter (ours, obviously) and is surprised to find intelligence. This confuses it as it usually only finds intelligent life in planes of energy. It drifts towards a life-form and is sucked into the head of a xenophobic American private soldier fighting in a future war against a Chinese invasion of the US. There's an incident where the soldier walks through a gas grenade without breathing apparatus. The Chinese (who're completely astounded by this) fire on the soldier and finally dispatch him with a grenade.
3. The entity, not realising that its host is a dying man (killed and dismembered by a grenade) begins to rebuild him out of dirt and individual atoms, eventually dragging itself out of the ground several years after the war has ended. He encounters a policeman and tells him to "Throw down your badge but keep your gun. You'll need it where we're going". The policeman, strangely mesmerised by his charisma, does as he's told.
4. Cut to a few years later. A group of insurgents/terrorists are watching the soldier (now President) give a speech. They have a psychic child who indicates that there's a second entity inside the President's head. The psychic shouts "Free!" and the entity returns to its own plane of existence. The soldier disappears in a puff of smoke.