When I mentioned to a diplomat that Jan. 14 would mark 30 years since Cambodian strongman Hun Sen became prime minister, his response summed up the pathology of forgetful international diplomacy: “Is that monster still around? How did that happen?”

As citizens of places such as Zimbabwe, Syria and North Korea have experienced, there are complicated reasons for the durability of despots. In Cambodia, it is a tale of international indifference, missed opportunities and an intelligent and ruthless leader who has run circles around internal party rivals, domestic political opponents and foreign governments.

Hun Sen has remained in power by manipulating elections, controlling the army and police, imprisoning critics and running a kleptocracy that has enriched him and his cronies. Ten years ago, a State Department official told me that the United States estimated his personal fortune at $500 million, attained largely through the plunder of the country’s rainforests and other natural resources.

Yet, as with many autocrats the world has tried to airbrush from its conscience, in recent years Hun Sen’s reign has continued almost unnoticed. As I visit foreign ministries around the world, I have found that most officials have become indifferent or even ignorant about the fate of a country that once was at the epicenter of geopolitics. This is ironic, as “the Cambodia problem” was once a primary impediment to untying the Gordian knot of diplomatic relations among the United States, China, the Soviet Union and Vietnam, all of whom were using Cambodia as a proxy (fighting “to the last living Cambodian,” as one observer put it). The solution was the 1991 Paris Peace Agreements, signed by leaders from Cambodia and 18 other countries and hailed as the beginning of a new era of democracy and human rights.

It hasn’t worked out that way. Democracy was virtually stillborn in 1993 when Hun Sen and his Cambodian People’s Party refused to accept defeat in a costly and bloody U.N.-administered election. He became “co-prime minister” for the next five years, and his party maintained control over the army, police and treasury — power it has never relinquished.

Hun Sen has never shied away from using violence to achieve political goals. Verifying detailed witness accounts, an FBI probe determined that his personal bodyguard unit was involved in a deadly 1997 grenade attack on an opposition political rally, killing 16. The story of the attack, which the United States officially deemed a terrorist act, led CNN and was on the front page of The Post. As a U.N. human rights worker, I was present at the gruesome scene, at which police refused to transport victims, some of whom bled to death on the street. The government has never pursued justice for the victims.

Soon after, Hun Sen staged a coup against his royalist coalition partners, using his security forces to execute more than 100 political opponents. Killings, torture, illegal land confiscation and other abuses have been the hallmark of Hun Sen’s rule to this day. Some 300 people have been killed in politically motivated attacks since the Paris Agreements. In many cases, the perpetrators are not only known but also have been promoted to high government or military posts. Unsurprisingly, not one senior official has been held to account.

A former Khmer Rouge commander himself, Hun Sen has used his control over the courts to mire U.N.-assisted trials of Khmer Rouge leaders in delays and obstruction. He could have been a hero, the man who brought the Khmer Rouge to justice, but he has stated that he would not allow further investigations beyond the handful of cases already filed. It appears that he is covering up for his own actions in the Khmer Rouge or those of senior colleagues in his party.

Every diplomat I’ve ever spoken to about Cambodia acknowledges that Hun Sen has blood on his hands and is highly corrupt. No one pretends that he is a democrat. Yet most governments turn away when he manipulates elections, arrests opponents and unleashes his security forces on peaceful protesters. As with many who stay long in power, he gives long, rambling speeches on national television and has begun comparing himself to great Cambodian historical figures while claiming exalted titles. His latest: Samdech Akka Moha Sena Pedei Techo, or “princely exalted supreme great commander of gloriously victorious troops.”

Many brave Cambodians risk imprisonment and worse while fighting for human rights and democracy. But they will fail without sustained and coordinated pressure from influential governments and donors.

The United States and many other governments talk about the “culture of impunity” in Cambodia, but it is time for them to also address their own culture of indifference. Otherwise, Hun Sen, who is only 62 and has said he wants to rule until he is 90, will continue to devastate his country for many years to come.

Brad Adams is Asia director at Human Rights Watch.