Cambodia's UN-backed court has found the top two surviving leaders of the Khmer Rouge regime guilty of crimes against humanity, sentencing them to life in jail.

Watch: Verdict announced in Khmer Rouge trials 

Here are some key facts about Cambodia's long road from horror to justice:

The Regime:

Led by "Brother Number One" Pol Pot, the communist Khmer Rouge seized control of Cambodia in April 1975 and immediately began dismantling modern society in its drive to transform the country into an agrarian utopia.

Up to two million people died from starvation, overwork, torture or execution under the regime, which abolished religion, schools and currency.

The Khmer Rouge was driven from power in 1979 by Vietnamese troops and former regime members who defected, including Hun Sen, now Cambodia's prime minister. Pol Pot died in 1998.

Hun Sen was a mid-level military commander until fleeing to Vietnam in 1977. Under him, the Cambodian government fought the Khmer Rouge until the movement collapsed in the late 1990s.

The Tribunal:

Cambodia and the United Nations signed an agreement in 2003 to establish the tribunal, known as the Extraordinary Chambers in the Courts of Cambodia (ECCC). Set up in 2006, it is a complex hybrid court combining elements of international and domestic law.

Its mandate is to prosecute senior leaders and "those most responsible" for the crimes committed between 1975 and 1979.

The court can impose a sentence of up to life in prison. There is no death penalty and no financial compensation for victims.

Funded almost entirely by foreign nations, the tribunal had spent about $215m by July 2014 and has faced chronic cash shortages.

It has also been dogged by allegations of political interference and a series of high-profile resignations.

The Accused:

"Brother Number Two" Nuon Chea, 88, and former head of state Khieu Samphan, 83, completed their first trial late last year on charges of crimes against humanity.

A second trial against the pair began last Wednesday in which they face charges of genocide of Vietnamese people and ethnic Muslims, war crimes and crimes against humanity.

Another defendant, former regime foreign minister Ieng Sary, died aged 87 last year while his wife Ieng Thirith, also an ex-minister, was freed from jail in September 2012 after the court ruled dementia left her unfit to stand trial.

In its historic debut trial, the tribunal in 2010 sentenced former prison chief Kaing Guek Eav, better known as Duch, to 30 years in prison for overseeing the deaths of 15,000 people. His term was increased to life after a failed appeal.

Those Who Escaped Justice:

"Brother Number One" Pol Pot died in 1998 at the age of 73 while under house arrest by Khmer Rouge rebels who had turned against him.

Ta Mok, a feared military commander nicknamed "The Butcher" for the massacres and purges he ordered, was arrested in 1999.

He had been awaiting trial by the court but died in 2006 at the age of 80.

Because of the tribunal's limited scope, thousands of lower-level Khmer Rouge members and fighters who carried out the regime's brutal acts will never face the court.

The tribunal is currently investigating two possible new cases, strongly opposed by the government, against several lower-ranking cadres, but there are doubts about whether they will make trial.