Although personal relations between Brian Mulroney and Stephen Harper have ranged over the years from cool to frigid, the former prime minister has generally avoided public censure of his Conservative successor.

Which makes Mr. Mulroney's criticism Wednesday of the Harper government's approach to the United Nations particularly interesting.

The UN, Mr. Mulroney told the Globe and Mail's editorial board, is "a vital instrument" of the Canadian commitment to multilateralism.

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"We don't have the strength to impose our will or get our way at all times," he observed. "We need the instruments of international harmony."

Mr. Mulroney was responding to the bare-knuckle speech to the General Assembly delivered by John Baird earlier this week, in which the Foreign Minister said Canada would no longer participate in "endless, fruitless inward-looking exercises," aimed at reforming the UN.

Canada, he said, would instead judge the organization by results – and then went on to list examples of cases where the UN had failed to achieve any.

Could there be a greater contrast than between Mr. Baird's blunt warning and the nuanced, diplomatic approach preferred by Joe Clark, Mr. Mulroney's foreign affairs minister?

"John Baird is an activist foreign affairs minister," Mr. Mulroney observed, "and I think in many areas he's doing a very good job."

But "something happened, when we were defeated at the Security Council, psychologically," he went on. While that defeat didn't affect the Harper government's commitment to multilateral diplomacy, he said, "I think it affected the level of enthusiasm."

Canada's bid in 2010 for a rotating seat on the Security Council lost out to Germany and Portugal.

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While the United Nations was far from perfect, Mr. Mulroney went on, "if you're going to be there, you should try to keep it as strong and as perfect as you can."

A gentle remonstrance, but a remonstrance nonetheless.

The relationship between the present and former prime minister has been up-and-down – with more downs than ups – for decades.

Mr. Harper, at the time a staffer for a former Progressive Conservative MP, joined the exodus from the party in the late 1980s that brought the Reform Party into existence.

More than a decade later, Mr. Mulroney worked behind the scenes with other influential conservatives to bring about the reunion of Reform – now renamed Canadian Alliance – and what was left of the PCs. Mr. Harper led the new Conservative Party to power in 2006.

But relations between the two men soon soured again, when Mr. Harper ordered the severing of all contacts among Conservatives with Mr. Mulroney during the public inquiry into the former prime minister's role in the notorious Airbus affair.

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Despite any personal animosity, Mr. Mulroney has avoided public criticism of his successor, perhaps aware of the damage that friendly fire can do to a political party.

But the United Nations is dear to Mr. Mulroney's heart. As he pointed out during the editorial board meeting, he appointed former Ontario NDP leader Stephen Lewis as a high-profile Canadian ambassador to the UN.

On his watch, Canada was ready-aye-ready when asked by the UN to send peacekeepers to trouble spots. And the Mulroney government was a leader in the ultimately-successful campaign to isolate the apartheid regime in South Africa.

The UN matters to Brian Mulroney a lot. And if his criticism of the Harper government's dismissive attitude was gentle and guarded, repeatedly qualified by such statements as "that was then and now is now, and I suppose life is different," the former prime minister was nonetheless clearly determined to make his opinion known.

This can hardly be expected to improve relations between the two men. Not that either seems to care.