The guy who ordered the preemptive strike was Gen. Jack Ripper (Sterling Hayden). After Slim Pickens rode the bomb, Turgidson wanted all the other B52s to strike Russia, but the Doomsday Machine was already started.
Were we ever sure just what the Doomsday Machine was? And was it a bluff, as the Soviet Ambassador was taking pictures of the War room while they were all arguing about Mine Shafts and such. I think it was a con.
Been awhile since I.ve seen the movie.