Milton Osborne's rebuttal to my piece on The worrying similarities between the Khmer Rouge and ISIS was a welcome prompt to flesh out my argument. Milton's wealth of expertise on Cambodia and the Khmer Rouge brings important nuance to my argument.
As Milton notes, David Chandler's excellent account highlights anti-Vietnamese feeling and resentment toward Sihanouk's consort's family as prime factors in his overthrow. I would cede to the scholarship of both on the root causes of his overthrow and more broadly on the motivations of the Khmer Rouge. As Milton rightly notes, the degree to which the bombing campaign impacted on the rise of anti-Vietnamese anger and desire for change is unknown. In its brevity, my post doesn't sufficiently acknowledge this fact.

Pol Pot with Nicolae Ceausescu, 1978. (Wikipedia.)
Between 1965 and 1968, 214 tons of bombs were dropped on Cambodia. This paled in comparison to the 2.75 million tons dropped on Cambodia until 1973. It may not be altogether surprising, then, that there were few rumblings of discontent in 1966, as Milton notes. That said, Kiernan and Owen's 2006 argument on the similarities between the bombing campaigns of Cambodia and Iraq still holds water, in that the destructive impact of the bombing campaign likely benefited Khmer Rouge recruitment. Milton and I disagree to the extent of this benefit to the Khmer Rouge, though we agree that it is extremely difficult to measure.
Similarly, in drawing parallels between the Khmer Rouge and ISIS, I have neglected mention of the immense impact of Prince Sihanouk in the coup and on the Khmer Rouge's rise. Prince Sihanouk, who was revered in the Angkor tradition as a deva-raja (god-king), fell out of favour with the army and the Cambodian right, who would lead the coup against him in March 1970. His policies were often contradictory, which may be explained by his attempts to keep his country neutral in the Vietnam War. He had allowed the Vietnamese to use Cambodian territory and Chinese arms were flowing from the Cambodian port in Sihanoukville to Vietnamese forces, including to the Viet Cong. Unsurprisingly, this angered many in the country and the US; the latter supported his ouster in favour of a pro-American government. In the fall of Sihanouk, we may glean some loose similarities with power vacuums and change in 'stability of the status quo' that emerged in post-Saddam Iraq.
Milton rightly points out the origins of the 'Year Zero', which was applied as a tidy catch-all name in 1978 in Ponchaud's book and the following year by a John Pilger documentary of the same title. The term itself draws reference from the French Revolution's turning back of the calendar in 1792. Pol Pot was exposed to both this revolution during his time studying in Paris and to Mao's Cultural Revolution during a trip to China in 1965 and 1966. The brutal 'Cultural Revolution' Pol Pot oversaw in Cambodia during the Khmer Rouge's reign have all the trimmings of the 'Year Zero' concept, even if this name was only assigned after the fact.
I certainly agree with Milton's final assertion that we need to highlight differences as much as the similarities. There is, of course, a wealth of difference between ISIS and the Khmer Rouge. Milton has rightly highlighted some of the details concerning the Khmer Rouge, yet the obvious differences between the groups should not dissuade us from searching for lessons from the heinous crimes of the past to stop crimes of the present and future. But exploring similarities in motivations (and to a lesser degree, actions) of past extremist actors can help inform counter-actions against current groups. 
This brings me to the underlying theme of my post. Both groups emerged as anti-progress movements in the shadows of larger geopolitical events. These permissive environments were the oxygen that birthed both groups. In my view, the success of their ideologies (short-lived in the case of the Khmer Rouge) was secondary; rather it was their free-riding on popular discontent that mattered most.
Most importantly, in drawing out similarities with past genocidal groups, we deflate some of the awe that surrounds current extremist groups. By remembering earlier 'big idea' extremist ideologies that failed, we tackle the narcissistic notions of those drawn to such groups expecting to participate in an 'end of time' event.
Lastly, comparisons between extremists groups can demonstrate that this is not a religious problem or an ideological one, but something far more profound and far more human. This helps us think more deeply about extremist groups such as ISIS and thereby gives us a stronger hand in tackling root causes of recruitment and the motivations of those joining such groups.