There was disturbing news in the cables from the Canadian diplomats in Harare. High-level sources had revealed to the Canadians that Robert Mugabe's soldiers were slaughtering thousands of dissidents in western Zimbabwe. The Canadian high commissioner called it a "reign of terror."

But the response from Ottawa was muted. A few days later, the high commissioner admitted to colleagues that his instructions from Pierre Trudeau's government were "very broad and soft."

When the government finally began to voice its concerns to Mr. Mugabe's officials, the tone was "amicable," a Canadian cable said. It politely described the massacres as mere "civil strife" – and it even agreed with Mr. Mugabe's complaint that the dissidents were a "problem."

Story continues below advertisement

Read more: Canada's African opportunity

The killing of an estimated 20,000 civilians in 1983 and 1984 by a Zimbabwean military unit trained by North Korea remains the most horrific atrocity of Mr. Mugabe's long rule – a crucial moment in his shift to dictatorship. But the Canadian reaction was so mild that the Trudeau government even invited Mr. Mugabe to visit Canada just a few months after the massacres began.

The previously confidential Canadian and U.S. cables, obtained by The Globe and Mail from scholars researching Zimbabwe, reveal new details of Canada's response to the killings.

They also offer important clues to the mystery of how the Zimbabwean autocrat was able to retain Western support during the early years of his often-brutal rule.

Mr. Mugabe, who took power as prime minister in 1980 after the demise of the white-minority Rhodesian government, deployed the army's notorious Fifth Brigade in early 1983 in a lethal campaign of retribution in Matabeleland, the stronghold of his political rival Joshua Nkomo. The operation was called Gukurahundi – "the rain that washes away the chaff."

It soon became clear that thousands of people were being killed. Many were burned alive in their huts or publicly executed after being forced to dig their own graves. Canadian teachers and staff at volunteer agencies were among those who witnessed the violence and reported it to the Mugabe government, which dismissed their concerns. (Many years later, after Mr. Nkomo's death in 1999, Mr. Mugabe referred to the massacres as a "moment of madness.")

Canada's first high commissioner to the newly independent nation, Robert McLaren, heard the reports and tried to raise the alarm. Before his death in 2015, he told a friend, veteran diplomat Jeremy Kinsman, that he had repeatedly informed Ottawa about the brutal attacks by the Zimbabwean military, but there was little interest.

Story continues below advertisement

"Ottawa wouldn't listen," he said, according to Mr. Kinsman's account. "No one wanted to know."

Mr. Kinsman, a former Canadian ambassador to the European Union and Britain, told The Globe that Mr. McLaren was dismayed when he realized that his internal reports about the violence were "not getting much traction back in Ottawa."

The reports may have been ignored because Zimbabwe's independence from white-minority rule in 1980 was seen as "more or less a good news story … and perhaps the wish was simply to keep it that way," Mr. Kinsman said.

The newly obtained Canadian diplomatic cables confirm that Mr. McLaren sent strong warnings to Ottawa about the 1983 massacres – but the reaction was sluggish.

The cables were obtained by historian Stuart Doran, author of the forthcoming book Kingdom, Power, Glory: Mugabe, Zanu and the quest for supremacy, 1960-1987.

In a cable on March 7, 1983, Mr. McLaren told his department that a Zimbabwean cabinet minister had informed him that the death toll was "in the many thousands." The diplomat described the violence as a "reign of terror" and urged Ottawa to protest to the Mugabe government through the channels of the Canadian aid program, which was spending millions of dollars in Zimbabwe.

Story continues below advertisement

But in a response two days later, the External Affairs Department (now known as Global Affairs) said the Canadian aid program in Zimbabwe should not be "put into play." It authorized a statement of concern to the Mugabe government, but it suggested that Mr. McLaren should refer to the massacres as "civil strife" and "conflict."

It also instructed Mr. McLaren to express some sympathy to the Mugabe government about the issues in the Matabeleland region, since Canada, as a multicultural country, could understand how "competing interests" might "threaten national unity." And it suggested a key concession to the government of Zimbabwe: "We are aware of problems posed by dissidents in Matabeleland."

Two days later, the department told Mr. McLaren that it had voiced its concerns in a "frank but amicable" conversation with Zimbabwe's high commissioner in Ottawa.

Another diplomatic cable, sent to Washington by the U.S. embassy in Zimbabwe on March 11, cited sources estimating that about 3,000 people had been killed by that point. But it quoted Mr. McLaren as saying he had received only "very broad and soft instructions" from Ottawa on how to express his concerns.

The U.S. cable also revealed that the Mugabe government was "furious" that a Canadian organization, World University Service of Canada, had told its volunteer teachers to leave Matabeleland because of the military atrocities, some of which had been witnessed by the teachers.

The U.S. cable was obtained by Hazel Cameron, a researcher at the University of St. Andrews in Scotland, through a freedom of information request. Her study of the international reaction to the massacres was published in the International History Review last month.

Story continues below advertisement

While Canada's reaction was weak, most other Western governments had a similarly restrained response. In her research, Ms. Cameron concluded that the British government had adopted "a policy of willful blindness" and even a "conspiracy of silence" about the killings.

In her 20-page study, she found evidence that the British high commission in Zimbabwe had consistently minimized the magnitude of the atrocities by the Fifth Brigade. One British diplomatic cable, for example, said Britain was sympathetic about the "difficulties" that the Mugabe government faced in "handling the dissident problem."

Despite the mounting evidence of atrocities, the British cables emphasized that Britain's main interests in Zimbabwe were economic and strategic: protecting British trade and investment in Zimbabwe, preventing Soviet gains in Africa and the "need to avoid a mass white exodus."

Mr. Doran concluded that the Western response was "late, tepid and oblique."

The expressions of concern by the Western diplomats, mild as they were, did eventually contribute to the "cumulative weight of pressure" from inside and outside Zimbabwe that persuaded Mr. Mugabe to "tone down" the intensity of the Fifth Brigade's violence in Matabeleland, he said.

"Mugabe did not stop the violence; he simply reconfigured it," Mr. Doran wrote in an article on his findings. "It adjusted its modus operandi, increasing the proportion of beatings to killings, and disposing of bodies in more furtive ways."

Story continues below advertisement

A few months later, Mr. Trudeau invited Mr. Mugabe to visit Canada. "He was wined and dined from Saskatchewan to Nova Scotia, met ministers, premiers and yes, the prime minister, without a word being said about these atrocities," said a commentary by Mr. McLaren and Art Wright, another former Canadian high commissioner to Zimbabwe.

"Canada and other like-minded nations, by refusing to address these early atrocities, missed an important opportunity to mitigate the human-rights violations," the two former diplomats wrote in the commentary, published in The Globe in 2005. "Had Mr. Mugabe been faced with the threat of international ostracism and the curtailment of substantial aid, it is unlikely that his ego or need for assistance would have allowed him to pursue his political objectives in this manner."

Ahead of Mr. Mugabe's visit, the Canadian high commission prepared a briefing note for Ottawa, which was obtained by Mr. Doran. It made no mention of the Matabeleland massacres except for a brief reference to "tensions and strife" and "human rights" issues. It predicted optimistically that the violence "will just be a bad dream" if Mr. Mugabe pursues his "policy of reconciliation."

After the visit, an internal analysis by External Affairs concluded that the visit was successful because Canada had avoided any criticisms of "tensions" in Zimbabwe and had instead expressed its concerns "in an indirect manner." This document, also obtained by Mr. Doran, included a summary of the meeting between Mr. Trudeau and Mr. Mugabe, which also made no mention of the massacres.

Four months later, the Fifth Brigade was sent back into Matabeleland, and a food embargo was imposed to control the population. Canadian diplomatic cables in April, 1984, cited by Mr. Doran described a pattern of "military repression, beatings, murders" and "enforced starvation" to crush the dissidents.

But this time the Mugabe government made a bigger effort to conceal the atrocities. The violence was concentrated "in hidden enclaves, away from prying eyes," Mr. Doran said.