Political assassination is a practice as old as human society, although the term itself derives from the 12th-century Persian Order of Assassins, first described by Marco Polo. Julius Caesar, Thomas Becket, Archduke Franz Ferdinand, Leon Trotsky, John F Kennedy, Lord Louis Mountbatten, Olof Palme and Yevgeny Prigozhin were victims of notorious political assassinations. They had one thing in common: all were high-profile targets.

That is not a description that may be accurately applied to Hardeep Singh Nijjar, a Canadian citizen shot dead in June by two masked gunmen outside a Sikh temple in British Columbia. If Nijjar had any claim to fame, it was as a campaigner for Khalistan, a notional Sikh homeland in the Indian Punjab fiercely opposed by India’s government. His activism provides the only plausible motive for his murder. Little-known though he was, Nijjar’s death was a political assassination, too.

After failing to obtain a private explanation, Justin Trudeau, Canada’s prime minister, went public last week, declaring that there were “credible reasons” to believe agents of the Indian government were behind the murder. His statement was immediately rejected by Delhi, which called the allegation “absurd”. That was a poor choice of word. A moment’s reflection should have told the prime minister, Narendra Modi, that it’s a very serious matter indeed.

Although Trudeau did not provide evidence for his claim, he would not have made it, in the formal setting of the Canadian Parliament, unless he had firm grounds for believing it to be true. It has emerged that the Five Eyes intelligence-sharing network – comprising the US, UK, Canada, Australia and New Zealand – may have provided Ottawa with incriminating information that allegedly points to the complicity of Indian officials and diplomats in Canada. If so, it would not be the first time India has been implicated in extra-territorial killings.

A less haughty, quicker-thinking figure than Modi would also have understood that Nijjar’s murder, appalling in itself, raised significant matters of state that Trudeau could not ignore. “Any involvement of a foreign government in the killing of a Canadian citizen on Canadian soil is an unacceptable violation of our sovereignty,” Trudeau said. After the poisonings by Russian agents in Salisbury, Britain knows how that feels.

Public expressions of concern by the US and UK governments were followed up in person when Joe Biden and other western leaders met Modi at the recent G20 summit in Delhi. This, too, should have persuaded Modi that, whatever the truth of the matter, he faced a damaging diplomatic row. Yet, unwisely, he escalated, expelling a Canadian diplomat and suspending visa services and trade talks. At this point, it is unclear where righteous indignation ends and purblind arrogance begins.

It’s true, as Indian critics say, that Trudeau faces domestic political complications. Canada’s sizeable Sikh minority wields significant influence. It’s also true that India has long regarded the Khalistan movement as a destabilizing separatist force supported by terrorism. Yet Modi, an authoritarian populist who tends to treat any opposition as a betrayal akin to treason, faces political complications of his own, principally a general election next year. Confronting Canada, a fellow Commonwealth country associated by some with the British imperial era, serves his Hindu ultra-nationalist agenda.

India is a rising power on the global stage that ostensibly shares western values. Britain and the US view it as an important ally in the wider contest with China. But the Modi government’s behaviour at home and abroad raise doubts about its commitment to democracy and India’s reliability as a partner. Nijjar’s assassination, like that of the Saudi dissident, Jamal Khashoggi, leaves a bloody stain that will be hard to wash away.                                                                                                                                  – Observer editorial