From the Others' perspective, in 1954 when the United States introduced a nuclear weapon onto the Island. It went into high gear when DHARMA introduced a "permanent occupation force" onto the Island.
As of "Some Like It Hoth" the identity of the side that fired the first shot is unknown.
By the time Sawyer, et. al. finished time shifting and ended up in 1974, a truce had been established between the Others and DHARMA.
By 1977 (possibly earlier), DHARMA had been granted access to areas of the Island for a variety of purposes, including construction of stations. Other areas (grids) were off limits.
The truce was violated on more than one occasion by both sides, but there were procedures for solving these problems.
One day, something happened that was too big to overlook.
If the occurrence of the Incident halted the ability for people to have children on the Island (Ethan Rom's birth being a marker for it being very possible pre-Incident), both parties would now rely on outside sources for further recruitment. Though the nature of recruiting Others remains a mystery (Post-Incident, but Pre-Purge), the process through which DHARMA recruits would only serve to tip the balance between the two factions.
It could have been ordered by Jacob as a preventative measure to keep the increasingly damaging experiments of the DHARMA Initiative from destroying or at least horribly damaging the Island.
Given the fact that Jacob when bringing people to the Island or not, never intervenes and makes it highly unlikely he had a part in the purge.
The Others were only protecting their own Island and after a long war went to extreme measures to wipe out DHARMA.
The DHARMA Initiative had learned that the Island's main function was to keep the Man in Black prisoner (Egyptian hieroglyphs in Swan Station's computer room). Despite their precautions to avoid any outside influence (computer use only to enter the Numbers) they had become a security risk.
Or the events in Ben's flashback may not be the "purge" Mikhail was referring to. This was an unrelated attack on one of the DHARMA Initiative Stations during a long war.
Ben's flashbacks confirm that there were hostilities between Dharma and the natives since he arrived as a child, so it is possible the purge was initiated before he even arrived on the island, or began sometime in between his flashbacks and the beginnings of the purge were never shown.
The Purge was likely a lasting conflict and not just the release of the gas in the Tempest. It seems that the Purge is a mini-war rather than an incident(not to be confused with The Incident)
The DHARMA staff took strong precautions against the Flame falling into the hands of The Others at the time of the Purge. The station was wired with large amounts of explosives and a special code triggering the demolition of the station if it were taken by the Hostiles.
DHARMA members had special weaponry developed by the Hanso Foundation.
DHARMA members had special abilities created, e.g., by genetic modification.
The DHARMA people were not run-of-the-mill scientists - that we know - but they were probably still scientists, albeit ones who, due to the ultra-sensitive nature of their work, required special training before being put on the Island. It is unlikely that the scientists themselves were genetically modified, but genetics may well have been part of their work.
This would suggest Kelvin has been alone in Swan for around ten years. This is possible but unlikely.
The Purge was a full assault on the DHARMA people, but the gas attack was not the entire assault, as there were other casualties. In the DHARMA grave, there is a distinct shot of a dead DHARMA body, with a gunshot wound in his skull, suggesting he was shot, not gassed, so there are likely other events that take place in the Purge in addition to the gas attack, which likely did not wipe out ALL DHARMA people.
He may have been gassed then shot. Historically it's not unusual for the dying to be "finished off" after an attack of this sort.
Date of the Purge
The date of the Purge is December 19, 1992. This is based on December 19 being Ben's birthday (from the reasoning for the Lostpedia timeline) and the purge happened on his birthday. It was in 1992 because Horace said, in a vision, that he had been dead for 12 years (Cabin Fever).
He was stationed there after the Purge. (Enter 77) We know many of his statements are untrue but this supports a 1992 Purge.
A problem with a 1992 Purge is the fact that Danielle's changed the radio tower message in 1988, with no one in DHARMA noticing for four years. (Pilot, Part 2)
Not necessarily. It was stated elsewhere that the radio tower had been abandoned by DHARMA for some time before the purge. If they abandoned it, they likely weren't monitoring it either.
Where is "elsewhere"?
The date of the Purge must be considered uncertain following the revelations in "Dead Is Dead." If Horace's apparition is the only evidence suggesting 1992 then there is more to learn.
Mikhail's statements are not inconsistent with the Purge being earlier than 1992.
Mikhail is an Other. There is nothing to lead us to believe that what he says is true. We have seen him lie in the past. Therefore we must believe the vision of Horace in Cabin Fever that the purge took place in 1992.
However, the DHARMA recruitment of Kelvin after the Gulf War suggests the purge was in the 90's.
The date of the Purge is December 19, 1988. The dream encounter between Horace and Locke could have also taken place in 2000. The vision is pretty inconclusive, especially given the fact that we see Horace working on his cabin which took place before 1992, and Mikhail's statements can't be taken at face value.
However Kelvin was recruited by the Dharma Initiative sometime after the first Gulf War, that would put it in the 90's.
According to the Truce between Richard Alpert and Horace Goodspeed the DHARMA Initiative was supposed to leave the island by August 15, 1988. Yet, even with an allowance of 5 extra months, DHARMA hadn't left but had actually dug in to prepare a strike against the Others. The plan backfired when the Others seized control and used the Tempest instead as a preemptive strike.
Danielle Rousseau and her team arrived after the Purge. While they thought they arrived in November, it had actually been in December (considering the time differences between the Island and the real world).
According to Lost Encyclopedia, the date was approximately 15 years after The Truce was signed, meaning late 1988.
According to the LOST Experience the Hanso Foundation stopped funding the DHARMA Initiative in 1987, possibly suggesting that the Hanso Foundation was aware of the deadline of the Truce. Unless somebody else provided funding for the next 5 years until 1992.
The Lost Experience's canonicity is suspect.
The Purge would have to be before Ben adopted Alex Rousseau, or else she would have died, suggesting it was before 1988.