In what has quickly disintegrated into a humanitarian disaster of historic proportions, a staggering 400,000 Rohingya Muslims have fled from Myanmar’s northern Rakhine state to Bangladesh over the past three weeks alone. At least 240,000 of them are children.

The Rohingya are fleeing a campaign of indiscriminate violence by Myanmar’s military, whose tactics are being widely condemned as a form of ethnic cleansing.

Entire villages have been burned to the ground. Women have been raped. Rohingya refugees report that soldiers shot at them as they fled. Along the border with Bangladesh, there are reports that the military has laid land mines to ensure those fleeing won’t return. Though independent observers have no access to the region, the Myanmar government now says 175 villages in the region — 30 percent of all Rohingya villages — are empty.

“We are hearing really horrendous stories of people who have survived by the skin of their teeth,” Paolo Lubrano, an Oxfam worker in Cox’s Bazar, a town on the Myanmar-Bangladesh border, told me by Skype Friday morning. Lubrano described “dire violence” and an enormous number of very young, and very traumatized, Rohingya refugees. Among those fleeing Myanmar, he added, are many pregnant women who have been walking for three, four, or even five days to find safety.

The military calls the campaign a “clearance” operation against an insurgent terrorist military group. They claim the crackdown is in response to a series of armed attacks on border police by Rohingya militants on August 25 that left 12 officers dead, the second such type of attacks in the past 12 months. But observers say that though armed Rohingya insurgents exist, their overall numbers are small, and they are poorly equipped. And the crackdown has affected the entire ethnic group.

Meanwhile, in a Facebook post on Sunday, Gen. Min Aung Hlaing, the head of Myanmar’s military, was dismissive: “They have demanded recognition as Rohingya, which has never been an ethnic group in Myanmar.”

Zeid Ra'ad al-Hussein, the United Nations high commissioner for human rights, has dubbed this crisis a “textbook example of ethnic cleansing.” Some are going further — saying the country is tipping toward crimes against humanity and even possibly genocide. On September 14, US Secretary of State Rex Tillerson spoke out about the “horrors that we are witnessing occurring in Burma.”

“This violence must stop; this persecution must stop,” he said. “It’s been characterized by many as ethnic cleansing. That must stop.”

The world has turned to Aung San Suu Kyi — a dissident turned political leader who won the Nobel Peace Prize in 1991 and was often likened to Nelson Mandela — for answers. Now that she has become Myanmar’s de facto top civilian leader, she’s being widely criticized for failing to speak out against the violence. She abruptly canceled a planned trip to New York for next week’s United Nations General Assembly as her critics grew louder. On Sunday, UN Secretary General António Guterres told the BBC, “She will have a chance to reverse the situation, if she does not reverse the situation now, the tragedy will be absolutely horrible.” He was referring to a speech she is expected to deliver on Tuesday.

Yet observers say the groundwork for the carnage was laid long ago. Thousands of Rohingya have lived in decrepit internal displacement camps in Myanmar for years. More than 400,000 Rohingya refugees were already living in Bangladesh; now those numbers are doubling. And across the region, the Rohingya have not had proper access to health care or an education in decades.

But who, exactly, are the Rohingya? And what is their relationship to Myanmar?

Colonialism hurts, long after the colonists have gone

The Rohingya Muslims earned the title the “world’s most persecuted minority” long before Suu Kyi assumed a position of power in government in 2015, five years after being freed from her 15 (out of 21) years under house arrest. The description is usually attributed to the United Nations, but it is so often repeated that it’s hard to trace its origin. Either way, the moniker has stuck.

A 2013 Harvard Divinity School study concluded: “Today, the Rohingya face discrimination in areas of education, employment, public health, housing, religious activity, movement, and family life.” That includes a mandatory two-child limit per Rohingya household — a restriction that is only applied to the Rohingya. They also suffer from onerous restrictions on freedom of movement and the freedom to marry. Rohingya must request the right to marry from the government, a requirement also not imposed on other groups.

Many reports on Rohingya persecution and marginalization begin with Myanmar’s 1982 citizenship law, which stripped the country’s 1 million Rohingya of citizenship, leaving them without access to health care or education. Waves of violence soon followed.

In Myanmar, even the word “Rohingya” itself is taboo: The country’s leaders do not use it, and some asked the international community not to use the name. Buddhist leaders instead refer to Rohingya as “Bengali” — in essence labeling them immigrants and foreigners from Bangladesh. They are not included among the 135 ethnic minorities officially recognized by the state. State leaders consider them foreign interlopers with no real ties to the country.

But independent researchers of the region say that’s not true. One Human Rights Watch report in 2000 noted there were waves of Rohingya migration to what’s now Myanmar in the late 18th century, the 19th century, the 1940s, the 1970s, and again in the 1990s.

And a 2015 Economist deep dive into the history of the Rohingya put their origins in the region even further back:

Muslims probably arrived in what was then the independent kingdom of Arakan (now Rakhine) as long ago as the 8th century. They were seafarers and traders from the Middle East, and were joined in the 17th century by tens of thousands of Bengali Muslims captured by the marauding Arakanese. Some were forced to serve in the king of Arakan’s army, others were sold as slaves and yet more were forced to settle in Arakan. “Rohingya” simply means “inhabitant of Rohang”, the early Muslim name for Arakan. The kingdom was then conquered by the Burmese army in 1785.

Simmering anger toward the Rohingya long predates modern Myanmar, according to Azeem Ibrahim, author of The Rohingya: Inside Myanmar’s Hidden Genocide. The Rohingya minority, he told me, sided with the British colonialists who ruled the country (then called Burma) during World War II.

The country’s Buddhist majority, on the other hand, cast their lot with the Japanese who invaded their country midway through the war. Burma was considered a strategic regional linchpin. The Burmese saw the Japanese invaders, initially, as a way out of British colonialism. (They weren’t, at least not immediately.) But when the country won its independence from the British in 1948, the Buddhists remembered — angrily — that their Muslim neighbors had supported the British.

Discrimination defined the last quarter of the 20th century

In 1962, a hardline Buddhist nationalist named Gen. Ne Win took power in a military coup and quickly began scapegoating the Rohingya.

The 1982 citizenship law was a blow to the Rohingya, but it wasn’t the first, or the last. By the 1990s, waves of Rohingya had fled for Bangladesh.

Still, the majority of the community clung to life in Myanmar’s Northern Rakhine state. In 2012, journalist Greg Constantine traveled to the region for the Pulitzer Center to report on the Rohingya, a stateless people caught between two countries. “The Rohingya of Burma face severe restrictions on the right to marry,” his ensuing report concluded. “They are subjected to forced labor and arbitrary land seizure; they endure excessive taxes and they are denied the right to travel freely.”

And that was before things got even worse.

In late May 2012, four Muslim men gang-raped and killed a Buddhist woman. That horrific crime became a spark for mass violence between the two religious groups and a brutal government crackdown on the Rohingya. A 2013 Human Rights Watch report found that around 125,000 Rohingya, and some local non-Muslims, had been forced to flee their homes for squalid refugee camps in Rakhine state. Children had been hacked to death. Many thousands of homes were burned. The report’s authors concluded the violence amounted to ethnic cleansing and crimes against humanity.

In retrospect, the crackdown was a dark harbinger of the military attacks that would take place in 2016, and then again over these past few weeks.

In 2014, New York Times op-ed columnist Nicholas Kristof visited Myanmar and walked through refugee camps still crammed full with Rohingya. The Times posted a brutal video, narrated by Kristof, titled “21st Century Concentration Camps.” The people he met there had no freedom of movement and little to no access to health care. Their existence hung by a thread. It’s very hard to watch.

That same year, Fortify Rights, a human rights advocacy group based in Southeast Asia, published a report that detailed the problem further. “This report,” they wrote, “provides evidence that protracted human rights violations against Rohingya result from official state policies and could amount to the crime against humanity of persecution. The documents obtained by Fortify Rights detail restrictions on movement, marriage, childbirth, home repairs and construction of houses of worship, and other aspects of everyday life.”

Then in early 2015, researchers from the US Holocaust Memorial Museum’s Simon-Skjodt Center for the Prevention of Genocide visited Myanmar on a fact-finding mission. They reported on the “human rights violations [that] have put this population at grave risk for additional mass atrocities and even genocide.”

“We saw firsthand the Rohingya’s physical segregation, which has resulted in a modern form of apartheid, and the devastating impact that official policies of persecution are having on them,” the resulting report from the museum explained. “We left Burma deeply concerned that so many preconditions for genocide are already in place.”

And yet even as these reports were being written, hopes for Myanmar’s future under Aung San Suu Kyi continued to swell. In September 2016, Suu Kyi visited President Barack Obama in the Oval Office. During her visit, Obama spoke glowingly of Myanmar’s progress. He reminded the press that when he entered the White House, she was still under house arrest. And now she was the de facto civilian leader.

Obama noted that “because of the courage and strength and resilience of the Burmese people, what we've seen over the last several years is a transition to elections, a representative legislature that still has significant constraints from the previous military government but is giving voice to the hopes and dreams of a new generation of Burmese people.”

He then offered the Nobel laureate a larger prize than praise: “The United States is now prepared to lift sanctions that we have imposed on Burma for quite some time,” he said. “It is the right thing to do in order to ensure that the people of Burma see rewards from a new way of doing business and a new government.”

In December 2016, he made good on that promise — even though, by then, the country was being widely vilified for a new round of violence against the Rohingya.

The situation for the Rohingya took a dramatic turn in October 2016

The current crackdown on the Rohingya was triggered by an August 25 attack on a police station in Myanmar by a small armed faction of Rohingya called the Arakan Rohingya Salvation Army (ARSA). The militants, mostly armed with knives and crude implements, killed 12 police officers.

It wasn’t the first time that the armed group had attacked the country’s security forces. In October 2016, ARSA killed nine Myanmarese border guards. The brutal government response caused some 74,000 Rohingya to flee over the border to Bangladesh. Many have continued to live in refugee camps in Bangladesh.

In February 2017, a devastating report from the United Nations Office of the High Commissioner for Human Rights said that Rohingya refugees had seen indiscriminate killings — including of children. Of 101 women interviewed, more than half had been raped. The army, according to refugee reports, deliberately set fire to homes, schools, and buildings, sometimes forcing members of the community into the burning structures.

On August 23, 2017, just two days before the current bloodshed began, the Kofi Annan Foundation issued a grim report on the prospect of new violence between Rohingya Muslims and the government: “Unless concerted action — led by the government and aided by all sectors of the government and society — is taken soon, we risk the return of another cycle of violence and radicalization.”

It was a tragically prescient prediction.

Why doesn’t the world know more about the Rohingya?

Human Rights Watch and Amnesty International have both published satellite imagery showing miles of burned Rohingya villages, but the true scale of this month’s military campaign won’t be known for quite some time. Aid workers cannot enter the region, and journalists have almost no access.

The Rohingya are also a largely invisible and unknown population. They’re not remotely as well known as other groups seeking citizenship or recognition like the Kurds or the Palestinians.

“The Rohingya are primarily fisherman and farmers,” Ibrahim told me. “They don’t have a spokesperson.”

Myanmar, by contrast, does — and Suu Kyi remains a figure many world leaders are reluctant to attack too harshly. They are conscious of the fact that the country’s fragile power-sharing agreement means she has no control over Myanmar’s military or security apparatus.

On Thursday, Secretary of State Tillerson called the current violence a “defining moment” for Myanmar’s new democracy. But he also spoke of continued support for Suu Kyi.

In the meantime, the Rohingya continue to flee, stateless and terrorized. Over the weekend, the number of new refugees passed the 400,000 mark.