Here's a WorldViews guide ahead of the vote:
A former Khmer Rouge commander who lost sight in one eye in battle, Hun Sen defected from the bloody regime in 1977 to a base in Vietnam, where he received training for a role in replacing the brutal government of Pol Pot. He returned two years later, serving as foreign minister in what was essentially a Hanoi-installed puppet regime. In 1985, he became Cambodia's prime minister and de facto leader.
The prime minister was contending with the remnants of a Khmer Rouge insurgency in 1992 when a U.N. peacekeeping operation — for the first time anywhere — took over the administration of the state and ran elections there, with the agreement of the Cambodian government. The United Nations was tasked with putting in place a new, democratic government that was to be a shining example of the international organization's role in the post-Cold War era.
Hun Sen's party did not win a majority in elections the following year, but he refused to step down from his post and instead entered a power-sharing agreement with Prince Norodom Ranariddh, the leader of an opposition faction. Four years later, he ousted the Cambodian prince in a coup and secured a full grip on power.
There have been a number of contested elections since, rife with accusations of fraud. Most recently, in 2013, the opposition Cambodia National Rescue Party (CNRP) almost won and, in its defeat, raised the specter of widespread cheating. It called on the international community not to accept the result. Thousands took to the streets in widespread protests over the following months, challenging the rule of Hun Sen's Cambodian People's Party (CPP), leading to at least one death.
What is at stake in Sunday's election?
More than 100 seats in Cambodia's National Assembly are up for grabs. The country's electoral framework establishes, in theory, a multiparty liberal democracy, and the party that wins most the votes ends up forming the government and ruling the country.
After what happened in 2013, though, Hun Sen has left little to chance. Among his first targets: the media. The government has shut down radio stations for airing independent news broadcasts by the U.S.-funded Voice of America, as well as Radio Free Asia and Voice of Democracy, giving Cambodians, particularly in rural areas, little choice but to rely on pro-government broadcasters for their news. Two former RFA journalists are in detention on espionage charges that many view as politically motivated. The independent English-language Cambodia Daily was forced to close in September over allegations of tax irregularities in what critics say was a thinly veiled attack on critical journalism, and the Phnom Penh Post was sold to a Malaysian investor with ties to Hun Sen.
Hun Sen has also ensured that this time, he is running almost completely unopposed. The Supreme Court dissolved the CNRP, the main opposition party, last year and locked up its leader, Kem Sokha, on trumped-up charges of treason, detaining him up in a remote jail near the border with Vietnam. It banned 118 key leaders of the CNRP from all political activities for five years, leaving them unable to run even under another party's banner in Sunday's vote.
The election commission — which is not independent — has touted the presence of other political parties as evidence that the election will be competitive and contested. But the 19 small, obscure parties are too insignificant to pose a real challenge to the CPP's dominance, analysts say. In a rally Friday marked by a flood of people in blue and white, the colors of the CPP, Hun Sen boasted of his success in eliminating “traitors who attempted to topple the government,” a reference to the CNRP, and urged party officials to continue to vote for their own.
Where is the remaining opposition, and what is it doing?
Much of Cambodia's opposition is abroad, in forced or self-imposed exile, fearing arrest and intimidation by the ruling party. From their bases across the world, they are calling on foreign governments to take more-decisive steps against Cambodia as it cements one-party rule. From his base in Paris, CNRP President Sam Rainsy has led a campaign calling for a boycott of the vote — a “clean finger” campaign in reference to the ink used to stain voters' index fingers.
The boycott calls have worried Hun Sen's government, which has to rely on voter turnout to prove that his victory is legitimate, since the ruling party is not running against a credible opposition. A low turnout, analysts and human rights groups say, would underscore just how bogus the election was. But they fear that local election officials could cheat and inflate the number of votes, and they also point out that a campaign of fear and intimidation is ongoing to ensure that people go to the ballot box — or risk losing their jobs, livelihoods and freedoms. Five former opposition members were fined the equivalent of $2,500, about twice the annual wage in Cambodia, for promoting the boycott.
How has the international community responded?
The United States and the European Union have refused to send monitors to Sunday's election or to fund it in any way. Japan, a major donor to Cambodia, said this week that it would not be sending monitors after all, though it has provided technical assistance. The specter of U.S. sanctions looms over Cambodia, after the House passed a bill that would seek to punish Cambodian officials in Hun Sen's inner circle who undermine democracy and commit human rights violations. The E.U. has warned that it could remove Cambodia from a preferential trading scheme.
But Hun Sen has a powerful ally in China, which has come to Cambodia's defense against these threats from the West. China is Cambodia's biggest investor, pumping money into infrastructure development such as roads and bridges. It marks a shift in the global power balance in Cambodia from the days of the United Nations, when Western countries were dominant and able to steer the Southeast Asian country's path through billions in aid and development funding.
Cambodian opposition leaders and activists want the international community to do more. In rallies across capitals planned Sunday, activists are calling on the international community to reject the results of the election and to not recognize the resulting government.
“Now it’s up to the countries that committed so much to the 1991 Paris Peace Accords to help restore genuine democracy to Cambodia or accept the human rights consequences of an effectively one-party state,” said Brad Adams, Asia director at Human Rights Watch.
Correction: An earlier version of this report said elections would take place June 29. They'll take place Sunday, July 29.