A little over a month ago, Canada and India were still negotiating a bi-lateral deal that would increase trade and expand investment between the two countries.

Now those talks have halted, Canada has implicated the Indian government in a murder on Canadian soil and, as of yesterday, India has ordered Canada to remove most of its diplomats from the country.

The origin of this growing tension goes as far back as the 1940s, and while relations between Canada and India are generally friendly and productive, a handful of events in the last 75 years or so have caused tensions to flare up periodically.

Here's how Canada and India got to this point and what's at stake.

HARDEEP SINGH NIJJAR KILLING

Last month, Prime Minister Justin Trudeau said there were "credible allegations" of Indian involvement in the murder of Sikh activist Hardeep Singh Nijjar, who was gunned down outside a Sikh temple in Surrey, B.C..

For years, India alleged Nijjar, a Canadian citizen born in India, had links to terrorism. Nijjar was a prominent member of a political movement to create an independent Sikh homeland known as Khalistan, but denied having any links to terrorists. At the time of his murder, he was working with the group Sikhs for Justice to organize an unofficial referendum among the Sikh diaspora.

The Indian government quickly denied any involvement in Nijjar's murder and the two countries have traded diplomatic barbs since then, most recently culminating in India demanding Canada reduce its diplomatic presence there.

Speaking to reporters after a Liberal caucus meeting on Oct. 3, Foreign Affairs Minister Melanie Joly said the two governments are in constant, private talks regarding an investigation into Nijjar's death and the diplomatic fallout.

"In moments of tensions, because indeed there are tensions between both our governments, more than ever, it's important that diplomats be on the ground and that's why we believe in the importance of having a strong diplomatic footprint in India," she said.

THE KHALISTAN MOVEMENT

According to Maika Sondarjee, assistant professor of international development at the University of Ottawa, Canada's friendliness – or lack of hostility – toward Sikh separatist activists has been a sore point for India for decades.

Sikh separatists associated with the Khalistan movement first began moving to Canada in large numbers after the partition of India and Pakistan in 1947. When the two countries were separated, the Punjab region, which is home to most of the world's Sikhs, was split into the Indian state of Punjab and the Pakistani province of Punjab.

While Sikhs form the majority of people living in Punjab, they form only two per cent of India's population of 1.4 billion. The modern movement to create a sovereign Sikh homeland called out of the state of Punjab emerged from negotiations preceding the partition, though Khalistan has never been recognized by the Indian government, which considers the movement a national threat.

"With the separatist movement came a lot of violence, and then with the violence, a lot of people left India," Sondarjee told CTVNews.ca in a phone interview on Monday. "And so since then, Canada has been a land of welcome for Sikh separatists historically."

In one of the Khalistan movement, the Indian Armed Forces in 1984 launched an assault on a number of Sikh holy sites in Punjab, including the Golden Temple, the holiest site of Sikhism. The raid, known as the Blue Star Operation, was intended to remove Sikh separatist militants from the sites. However, it resulted in the deaths of at least 493 civilians.

Sondarjee said Canada has never "strongly condemned" the Khalistan movement, and since much of the movement's strength comes from the efforts of the Sikh diaspora in countries around the world, the Indian government views Canada as accepting, if not outright supporting, the movement.

"I'm pretty sure Justin Trudeau would never say we are supporting the separatist movement for Khalistan," Sondarjee said. "But the fact that we accept a lot of refugees coming from that movement, a movement that is considered terrorist by the Indian government, that's—for them—a symbol of our acceptance."

The Khalistan movement is still active today. Organizers of an unofficial worldwide referendum on Punjabi independence conducted the first stage of a series of non-binding votes in British Columbia last month, Nijjar had been one of those organizers before his death

TERRORISM ALLEGATIONS

While the Indian government considers the Khalistan movement itself to be a threat to national security, it has also condemned a series of retaliatory attacks by Sikh separatists over the years, at least one of which was linked to Sikhs in Canada.

Following Operation Blue Star, India's Prime Minister Indira Gandhi was assassinated on Oct. 31, 1984 by two of her body guards, both of whom were Sikh.

The next year brought the bombing of and the deaths of all 329 people on board. A bomb exploded on the passenger jet on June 23, 1985 while it was en route from Toronto to London, England. Most of the passengers killed were Canadian, and to this day, the bombing remains one of the worst terrorist attacks in Canadian history.

Although several people were arrested and tried for the bombing, only Inderjit Singh Reyat, a dual British-Canadian national, was convicted.

"The suspicion was that those [tried] were Sikhs seeking revenge for Indian repression," McGill University international law professor Frederic Megret told CTVNews.ca in a phone interview on Tuesday.

"And I think that really put on everyone's radar that there was a kind of transnational dimension to this, and that it was particularly worrying to the Indian government that some of these individuals were operating from Canadian territory."

The Indian government has since labelled many Sikh separatist organizers in Canada, including Nijjar, terrorists. On Sept. 21, Indian External Affairs Ministry spokesperson Arindam Bagchi criticized Canada's "growing reputation as a safe haven for terrorists, for extremists, and for organized crime."

Last June, India's External Affairs Minister Subrahmanyam Jaishankar condemned images of a parade alleged to have occurred in Brampton, Ont. which appeared to portray Gandhi's assassination.

Protesters in Vancouver

India has long argued that Canada's approach to Sikh separatists goes too far, but Megret said the freedom of expression laws that allow Sikh activists to demonstrate here are the same laws that apply to all activists in Canada.

"Sometimes manifestations of freedom of expression can be sort of rowdy or hostile here," he said.

"But the truth is we don't have laws to repress (free speech)…so we're basically stuck with our liberties and our commitment to a free and open society, whether foreign states like it or not."

WHAT'S NEXT AND WHAT'S AT STAKE

Despite episodes of tension over the years, Canada and India have managed to maintain a healthy relationship with strong diplomacy and shared commercial interests, Megret said.

"India is Canada's ninth largest trading partner," he said. "There's a lot of Canadian companies in India and new talks to make the relationship closer, so I don't see any sort of major incidents apart from the question of Sikh separatists as a recurring source of tension."

If the stalled trade deal with India should fall apart altogether, Canada would lose more ground than just with India. The deal is considered key to Canada's Indo-Pacific strategy, which would see this country in the Indo-Pacific region.

Because of this, Sondarjee said Canada needs India more than India needs Canada, and the Indian government's decision to expel 41 Canadian diplomats shows it knows that.

According to Megret, in order for Canada to regain the upper hand in this conflict, Canadian intelligence will need to provide credible, compelling evidence that Nijjar's killing originated with the Indian government or India's foreign intelligence agency.

If it fails to do so, he said, Canada’s credibility could take a major hit and diplomatic relations between the two countries could risk being severed entirely.

"Either Canada will be found to have made this up or to have seriously erred in suggesting that India was responsible, or India will be found to have done it," Megret said. "If the evidence is indeed credible, I think it probably won't do wonders for Indian-Canadian relations, but it would indeed shrink the space of deniability for India."

– With files from Reuters, The Canadian Press and The Associated Press