In the novel
Smith revealed the nature of his vision in a posthumous letter to his father, after attempting to assassinate Stillson at a town hall meeting in New Hampshire. Smith's attempt failed, but in his panic Stillson used a child as a body shield, a moment that was photographed and published. Stillson's political career came to an end as a result of the scandal, effectively preventing Armageddon.
In the film
In a future vision by Johnny Smith, he forsees Stillson being elected president and sees him at Camp David, yelling at the high members of his cabinet that they must start a nuclear war so that he can fulfill his 'destiny' and go down in history. His cabinet members see him as insane, but only do it after attempts on their life is made. His vice president asks God to forgive him as he punches the code into the device. However, in the aftermath of the scandal where he used a baby as a human shield, Stillson commited suicide with a pistol to the mouth, ensuring that none of his machinations would come to fruition.
In the TV series
Smith experienced his first vision of Armageddon while shaking Stillson's hand at a Faith Heritage Alliance rally along with Eugene Purdy. He subsequently revealed the nature of his visions to his physical therapist, Bruce Lewis, and began constructing an intricate map of incidents and individuals linked to the coming destruction. Smith later met a fellow psychic who "read" the date of December 31st, with no year, from him, and Smith surmised that to be the date of Armageddon.
With the deaths of Malcolm Janus and Walt Bannerman, Smith no longer experienced visions of Armageddon when touching Stillson. Smith inferred that with Janus no longer pulling the politician's strings, Stillson would no longer follow the course of events and decisions that would lead to Armageddon. Later, however, it appeared that Bannerman's death, and not Janus's, was the key to stopping Armageddon; with Bannerman's death, Sarah Bracknell became single again, and Stillson began pursuing her romantically. The two grew closer, until Stillson later lashed out in a rage, hitting Sarah's son J.J. and knocking him to the floor.
The contact caused J.J. and his biological father, Smith, to experience simultaneous visions of Armageddon again. The implication thus seemed to be that Sarah was key to stopping Stillson from allowing Armageddon, possibly by becoming close enough to the man to find concrete evidence that could derail his political career.