So modern Cambodian history is a little bit of a mess. Really, there's two questions here: why did the west support the Khmer Rouge, and why was Pol Pot not brought before a tribunal? Apologies for the reams of context, but I think it's important to understand what exactly was happening in Cambodia. Let's start at the beginning:

Prince Norodom Sihanouk

Sihanouk was the former King of Cambodia. Chosen by the French, he ruled as king until 1955, when he stepped down to participate more directly in post-independence politics. He created the "Sangkum Reastr Niyum", a political movement that behaved as a party. Sihanouk governed Cambodia autocratically. He suppressed left-wing activists but fostered strong ties with the communist bloc, especially with China. By the end of the 1960s, he started to lose authority in the country and he was overthrown in a coup in 1970 by Lon Nol, who founded the Khmer Republic.

After losing power, Sihanouk went on exile, mostly in Beijing and Pyongyang, and he called on Cambodians to join the Khmer Rouge to fight against the Lon Nol regime, a regime he believed was sponsored by the US. In combination with mass American bombings in the Cambodian countryside during the Vietnam War, the Khmer Rouge were able to recruit heavily from the peasantry who saw Sihanouk as a god-king (in line with traditional Cambodian conceptions of the monarchy). When the Khmer Rouge took power, Sihanouk was given the role of head of state, but didn't much like what he saw; he'd spend most of the Khmer Rouge period under house arrest in his palace.

The Khmer Rouge and Vietnam

The Khmer Rouge were virulently nationalistic and xenophobic. One noteworthy story is how a US-trained armed group of Khmer Krom, or ethnic Cambodians from southern Vietnam, crossed the border into Cambodia and sought to join with the Khmer Rouge. Theoretically, this group would have been very useful - they were well-armed, well-trained, and experienced. Instead, the group was told to disarm, promised a meeting with the local Khmer Rouge leader, and promptly gunned down. They were supposedly too Vietnamese.

Anti-Vietnamese sentiment has been prominent in Cambodia since the pre-colonial period, when Cambodia was at various points effectively a province of the Vietnamese empire. These sentiments were stoked further during the colonial period, when the Vietnamese acted as administrators across Indochina; they were prominent in Cambodia. Under the Khmer Rouge, like other minority groups, the Vietnamese population was subject to what effectively amounted to genocide - deportations and ethnic cleansing.

From 1977, the Khmer Rouge had initiated small-scale conflict between themselves and the Vietnamese. By 1978, things had escalated; an invasion by Vietnam at the end of 1977 eventually amounted to nothing. In 1978, paranoia amongst the Khmer Rouge grew, and the KR crossed the border into Vietnam and massacred civilians. Finally, at the end of the year, Vietnam launched a full invasion that toppled the Khmer Rouge regime. In its place, Vietnam installed their own communist regime called the People's Republic of Kampuchea (PRK).

So why did the US support the Khmer Rouge?

There's two main things going on here: firstly, opposition to Vietnam, and second, support for a new Cambodian coalition.

There were fears that Cambodia would be absorbed into an Indochinese union with Vietnam at its head. The PRK was effectively a puppet regime of Hanoi, with a strong Vietnamese military presence in the country. The Sino-Soviet Split, meanwhile, had created two communist camps, one led by the USSR and one led by China. Prior to the invasion, the Vietnamese had aligned itself with the USSR, while the Khmer Rouge continued to be backed by China. As such, despite the genocidal regime of the Khmer Rouge, both the west and China were not keen on the new Vietnamese regime installed in its place. This was the prime reason for the opposition to the PRK taking Cambodia's seat at the UN.

But it was also clear that the Khmer Rouge weren't the only armed group operating in Cambodia. By 1981, four groups had emerged that opposed the Vietnamese invasion of Cambodia:

• The Khmer Rouge - following the invasion, they retreated to the Thai border where they continued their armed struggle against the PRK. By 1981, they had formally renounced communism, and in 1985, Pol Pot had left leadership of the KR to Khieu Samphan (who himself was deeply unpleasant, and would eventually be tried for war crimes - and Pol Pot still drove things).
• FUNCINPEC - an abbreviation of the French for "National United Front for an Independent, Neutral, Peaceful and Cooperative Cambodia", Sihanouk had formed this group in 1980 as an explicitly royalist armed faction against the PRK.
• The Khmer People's National Liberation Front (KPNLF) - formed by former Prime Minister under Sihanouk Son Sann and a Lon Nol general named Dien Del, this group was formed in 1979 to oppose the Khmer Rouge; eventually, it would oppose the PRK.
• MOULINAKA - this armed group was formed in 1979 as a pro-Sihanouk organization.

In 1982, these four groups would form the Coalition Government of Democratic Kampuchea (CGDK) with Sihanouk at its head. It was this organization that received support from the west and China. Nonetheless, it was the Khmer Rouge who military dominated the CGDK.

So, in sum, the west supported the Khmer Rouge because they weren't Vietnam, who they feared would dominate Indochina. But it wasn't just the Khmer Rouge they were supporting - they were also supporting other armed groups and a coalition government, though a coalition government dominated by the Khmer Rouge.

Why wasn't Pol Pot tried in an international tribunal?

There's a couple of key factors to answer this question.

Firstly, the civil war between the PRK and CGDK had developed into a stalemate. It was clear that a peace settlement would not be the result of one party dominating the other, but that all parties would have to negotiate - including the Khmer Rouge. The Paris Peace Accords of 1991, which, among other things, laid out plans for 1993 elections and temporary UN governance of Cambodia, featured the full participation of the Khmer Rouge. It was anticipated that the Khmer Rouge would also take part in the elections.

As it turns out, the Khmer Rouge boycotted the elections and attempted voter intimidation at the ballot box. Even after 1993, the Khmer Rouge kept fighting the new Kingdom of Cambodia (with Sihanouk as king), and were periodically fairly successful. The new government of Cambodia was an uneasy peace between FUNCINPEC, led by Sihanouk's son and the victor of the 1993 elections, and the Cambodian People's Party, the descendant of the Vietnamese-backed party of the PRK who came second and still had major structural advantages in the new state. Simply put, the new Kingdom of Cambodia could not militarily defeat the Khmer Rouge and did not have the capability to put Pol Pot on trial (a state that led to this bizarre 1994 interview with Sihanouk, who suggested he take power and form a coalition government with the Khmer Rouge).

The defeat of the Khmer Rouge was partly the result of factional conflicts and war exhaustion. In 1996, Ieng Sary, the former minister of foreign affairs, surrendered with about half of the Khmer Rouge's soldiers. What was left of the Khmer Rouge began factional infighting, and the Cambodian government negotiated with the remnants of the organisation to try and finally resolve the conflict. Pol Pot died in 1998, but the final elements of the party did not surrender until 1999. So there was no real way to try Pol Pot to begin with.

As for the will to try Pol Pot - this sort of crosses the 20 year cut off, but there was and still is a lot of political opposition to Cambodia to try the remnants of the Khmer Rouge. Hun Sen, the current Prime Minister (and has been since 1985), was a former Khmer Rouge soldier who defected to Vietnam and may have participated in a massacre. Much of the leadership of the dominant Cambodian People's Party had participated in the Khmer Rouge at some point. There have been numerous issues with the Khmer Rouge Tribunal since its founding in 1997, partly due to political interference - the first indictment was in 2007, ten years after the founding of the court.

So, in short - Pol Pot never faced a tribunal because Cambodia was unable to capture him, seeing as they were still fighting a civil war with him and the Khmer Rouge right up until his death, and there may not even have been the political will to try him.

Give me about five minutes and I'll try and post some of my sources in a reply to this.