An ex-member of FOX HOUND, Snake was trained to be a great killing machine. After taking down the second Metal Gear and Outer Heaven from Metal Gear 2: Solid Snake, he retired to Alaska somewhere. Snake was sort of abducted and taken to a US Submarine to when the Terrorists took over Moses Island. He's the perfect man for the job.
Unknown to Snake and to the player until later in the game, (in other words this next part gives away a major plot twist so READ AT YOUR OWN RISK) Cambell never really expected Snake to make it and beat everyone. So they injected him with a virus called Foxdye to kill off almost everyone there!