Something that is notable about Communist* movements in various countries is that they, at least early on, often enjoyed grassroots support from much of society. Much of the Chinese peasantry, for example, seems to have genuinely supported and benefited from the early policies of the Communist Party (such as formerly landless peasants receiving their own land) which helped them to gain power. It is also worth noting that, egregious crimes and abuses of power notwithstanding, many communist regimes seem to have resulted in a general increase in standards of living for a lot of the population (though obviously there were also times when their policies inflicted immense suffering) which makes it understandable why some of the population would have supported them despite the generally authoritarian nature of these regimes.
None of this seems to be true of the Khmer Rouge. Their ignoble reign seems to have made virtually no positive contribution to the livelihood of anyone who fell under their control. Regarding the peasants whom they envisioned as leading the perfect form of human existence, they seem to have placed extreme demands of increased productivity upon them from the outset as well as forcefully and radically restructuring rural society to be under their firm control with little to no room for independent living. Meanwhile urban populations were forcefully deported from their homes, subject to immense violence and forced to live on farms where starvation awaited them. For Cambodia as a whole, a period of less than four years saw some one quarter of the population dead, about half of them seemingly having been executed.
I guess what I'm asking is how such a regime could come to power in the first place? It is my understanding that even the worst of governments can't take over a country without having some kind of firm support base, but I simply can't imagine anybody in Cambodia thinking that the Khmer Rouge were good for them. We're they a lot less...radically crazy during the civil war than they were after capturing Phnom Penh? (The Chinese Communists seem to have become progressively less conciliatory towards people deemed class enemies as time went on, being rather more moderate during the war). Was there any major part of Cambodian society for whom the Khmer Rouge government was beneficial?
The Khmer Rouge also seem to have lacked many of the trappings of power seen in other Communist regimes. There was, for example, no major personality cult, most people didn't even know who Pol Pot was at first, and the entire party ruled in secret, presenting itself simply in the guise of "Angkar." What was it that put the Khmer Rouge in a position to take over Cambodia, given how their political programme of essentially enslaving the entire population on state-owned farms could hardly have been appealing to many?
*(Lumping such an idiosyncratic regime as the Khmer Rouge under the communist umbrella seems debatable, especially given that most of what they stood for - disdain for the urban working classes, reverence for an idealised Angkor Empire, extreme nationalism and ethnosupremacism, agrarianism and a belief that the peasantry are not only a revolutionary class but should be the only class - seems to fly right in the face of Marxism, but they seem to have at least originally considered themselves communists and are conventionally referred to as such.)
Initially the Communist Party of Kampuchea (CPK) found support in many quarters, in Phnom Penh as well. Many Cambodians had become appalled by the state of their country before the civil war began, and this impression only got worse as the brutal conflict wore on. Many urbanites dismissed as propaganda the stories about the Red Khmer atrocities. In the countryside, rural populations had often been neglected or taxed exorbitantly, so a new society where they would occupy more powerful positions was naturally appealing.
Cambodia’s population was overwhelmingly rural, and would go on to form the ‘base’ people in the new revolutionary society. However, even by 1972, there were massive areas of Cambodia already under the control of the CPK. These populations were not particularly used to doing much than subsistence agriculture, so living under the CPK wasn’t that different. This is where the CPK gathered most of their support, particularly in the young and the very poor, who were often taken from their homes and subjected to indoctrination courses. These populations were easily sold this idea that they had been exploited, chi cho’n ‘ridden and kicked’ in Khmer.
The fact that Sihanouk had ‘defected’, (more or less) to the cause of CPK was also a massive injection of support for their movement. The Khmer Republic was a shambles, one civil engineer in Phnom Penh thought that ‘whatever had been going on in the countryside it could not have been worse than the disorder and corruption of the Khmer Republic’.
The widespread ‘prison farm’ that Cambodia became after 1975 was not exactly what had been sold to any particular part of the society. Many skilled and educated people in the city assumed that their positions and some of their privilege would still remain, even if revolution was necessary to rebuild the country after such a difficult period. You ask how they came to power at all, well, as they were the only political group fighting against the Khmer Republic, it basically came down to the Lon Nol regime’s collapse that led to the Pol Pot regime. But they had enough support in the countryside to be able to achieve this, and don’t forget they employed terror on a large scale in order to maintain that power. Office 13 (M13) was a security prison ran by Duch (the commandant of S21) as early as July 1972, and its operations were very similar to that infamous place that would become synonymous with the regime.