In 1974, India shocked the world by detonating a nuclear device, drawing outrage from Canada, which accused India of extracting plutonium from a Canadian reactor, a gift intended solely for peaceful use.

Relations between the two nations cooled considerably – Canada suspended support to India’s atomic energy programme.

Yet neither expelled their top diplomats like they did on Monday as the row intensified over last year's assassination of Hardeep Singh Nijjar, a Canada-based Sikh leader labelled a terrorist by India.

The tit-for-tat expulsions followed PM Justin Trudeau’s claim that Canadian police were investigating allegations of Indian agents’ - and the Indian government's - direct involvement in the June 2023 killing.

Canadian police further accused Indian agents of involvement in “homicides, extortion and violent acts” targeting pro-Khalistan supporters advocating a separate Sikh homeland in India. Delhi rejected the allegations as "preposterous".

There are some 770,000 Sikhs living in Canada, home to the largest Sikh diaspora outside the Indian state of Punjab. Sikh separatism - rooted in a bloody insurgency in India during the 1980s and early ’90s - continues to strain relations between the two countries. Canada has faced sharp criticism from Delhi for failing to oppose the pro-Khalistan movement within its borders. Canada, says India, is aware of local Khalistani groups and has been monitoring them for years.

A mural features the image of late Sikh leader Hardeep Singh Nijjar, who was slain on the grounds of the Guru Nanak Sikh Gurdwara temple in June 2023, in Surrey, British Columbia, Canada September 18, 2023. 

A mural features the image of Hardeep Singh Nijjar, who India labelled a terrorist "This relationship has been on a downward trajectory for several years, but it's now hit rock bottom," Michael Kugelman of the Wilson Center, an American think-tank, told the BBC.