Diplomacy is full of unwritten rules, such as: When visiting a foreign country, do not appear to be cozy with a notorious ex-terrorist who tried to assassinate a cabinet minister from that country.

Okay, that one might actually be written down. In either case, the Trudeau government ignored the rule this week.

In a spectacular gaffe that exacerbated an already troubled week-long excursion to India, the Canadian High Commissioner in India invited a man named Jaspal Atwal to an official dinner with Prime Minister Justin Trudeau.

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Mr. Atwal once belonged to a group of Sikh separatist terrorists and was convicted in Canada of the attempted murder in 1986 of Malkiat Singh Sidhu, a moderate Sikh politician and Indian cabinet minister who was visiting British Columbia at the time. Mr. Atwal's invitation was rescinded only after the CBC asked the PM's office about it.

This would be bad enough in a vacuum. But India is already preoccupied with Sikh separatism in Canada, and with the Trudeau government's apparent willingness to tolerate it for votes.

Last spring, the Indian government objected to Mr. Trudeau's appearance at a Sikh community event in Toronto that reportedly featured floats celebrating extremists. The issue has shadowed this trip from the beginning. Which makes it all the more stunning that the government should have let this happen.

Did the name not ring any bells when Mr. Atwal was added to the invite list, apparently by a Liberal backbencher from B.C.? Or when Mr. Atwal posed with Sophie Grégoire Trudeau and Infrastructure Minister Amarjeet Sohi at an event in Mumbai on Tuesday? Couldn't someone have googled him?

Yes, the PM quickly admitted the invitation was a mistake, and Indian Prime Minister Narendra Modi graciously tweeted on Thursday that he was looking forward to meeting with Mr. Trudeau on Friday.

But what a mess. Yesterday, we quaintly said the India trip seemed awkward and unproductive. Those were the days.