Many of Stéphane Dion's 21 years in federal politics were spent in a cruel spotlight that exposed weaknesses that, in some ways, were at the root of his many successes.

In the statement he released to reporters after being replaced as Canada's foreign affairs minister, Dion said he entered politics at the request of Prime Minister Jean Chrétien on Jan. 25, 1996, "envisaging a brief parenthesis" in his life.

"It has been, in fact, an incredible adventure."

By all accounts, Dion didn't see today's cabinet shuffle coming and found it hard to accept that he was being asked to move on.

"You have to be able to read the tea leaves," said former interim party leader Bob Rae.

"I know he was thrilled to be the minister of foreign affairs and I think he felt that that was the high point of his public life and I think to lose that job was tough for him."

According to Rae, it would have been a challenge for Dion to stay on as foreign affairs minister with Donald Trump's administration on the way in the United States.

Dion stands behind interim Liberal Leader Bob Rae in 2011. Rae said Tuesday it would have been difficult for Dion to stay on as foreign affairs minister with the incoming administration of Donald Trump in the U.S. (Sean Kilpatrick/Canadian Press)

"Given Mr. Dion's own style and his own commitments on climate change and everything else, I think just the chemistry would have been difficult for him with the former president of Imperial Oil as the secretary of state in the United States," Rae told CBC News.

That style — or lack of it, depending on your point of view — is why some of those close to Dion felt his appointment as foreign affairs minister wasn't the perfect fit for the former academic.

When Marlene Jennings talks about Dion, her voice fills with tenderness. The former Montreal-area MP entered politics a year after Dion and calls him a kind and wise friend.

"On the one hand, in terms of intellectual knowledge, real knowledge of what's going on in the world, there are few who could surpass Stéphane on that," she said. "But it has the other aspect, which is being able to make that human connection and develop friendships easily, even if they're only facades, if it's, you know, only a diplomatic friendship. Those are not things Stéphane does easily."

As the 1992 referendum approaches, the future Liberal leader discusses Quebec's push for power. 0:31

Over the years, many who worked with and for Dion commented on his brusque manner and how he could be quick to dole out harsh criticism.

"When you want to speak to Stéphane Dion to put forward an argument in favour or opposed to a particular issue, you need to get up early and do your homework because if you showed up with half-assed arguments, he'll shut you down really quickly," Jennings said.

Peter Donolo, who served as chief of staff to former Liberal leader Michael Ignatieff and before that as Chré​tien's director of communications, says Dion is an indispensable go-to guy who is relentless when given a task, to the point where he has "no room for pleasantries or niceties."

"If the most important element in political success is EQ [emotional intelligence], that's kind of like the one ingredient that he was missing but he made up for it in pure intellect and in integrity and determination," Donolo said.

Chrétien had Dion draft Clarity Act

Chrétien called on Dion to enter federal politics soon after the narrow federalist victory in the 1995 referendum on Quebec sovereignty.

"He went into public life because of his integrity, because of his feeling that the country was in crisis and he responded to a prime minister reaching out to him in a real moment of crisis for the country," Donolo said.

Dion, a prominent and outspoken political scientist who was affectionately teased for always wearing a modest leather backpack, became Chrétien's minister of intergovernmental affairs. Within months, he won a byelection in the Montreal riding of Saint-Laurent-Cartierville.

Dion speaks before a Senate committee studying the Clarity Act back in 2000. (Jonathan Hayward/Canadian Press)

He wrote the Clarity Act to establish rules and conditions for any future attempts at secession. Jennings says Dion wasn't very popular in the party initially.

"He was viewed, when I came in, with some hostility by a significant number of MPs and senators from Quebec, and from some of the other provinces, because they felt he was, you know, he was a Johnny-Come-Lately, he hadn't been involved in the Liberal Party. He hadn't paid his dues," she said.

Not only that, many of those Quebec Liberals didn't want the federal government to push for the Clarity Act, fearing it could rekindle separatist embers.

Yet the Supreme Court backed Dion's position that Quebec could not unilaterally secede from Canada and the bill passed into law in 2000.

Champion of green politics

A Chrétien loyalist, Dion didn't make it into Prime Minister Paul Martin's first cabinet but, just as he did in later years, when his stock was low, Dion remained a loyal, hard-working member of caucus.

He returned to cabinet in 2004 as a passionate environment minister. He named his dog Kyoto after the United Nations climate change protocol signed in 1997.

Dion presided over tricky climate change negotiations in Montreal in 2005. Even Green Party Leader Elizabeth May called Dion a "very, very good environment minister."

If Dion's political career was divided into three parts, his second act would be as the unlikely leader of the Liberal Party.

Again, the intellectual who struggled to express himself in English surprised the country by winning on the fourth ballot, beating out front-runners Bob Rae and Michael Ignatieff.

Dion, surrounded by Jean Chrétien, left to right, Bill Graham, Joe Volpe, Mike Eizenga, Paul Martin and John Turner, gets tangled up in streamers following his speech after winning the Liberal leadership in 2006. (Canadian Press)

Dion's mostly overlooked campaign was based on three pillars: social justice, economic prosperity and environmental sustainability.

"In a sense he was the accidental leader of the party," Donolo said. "I mean the two front-runners — one was seen to be an American and the other one was seen to be an NDPer, so [Liberals] turned to the one they were familiar with and who had been in the battles."

'It's because I failed'

With the leadership came an enormous target on Dion's back.

"It was a difficult period for him," Donolo said. "He was really the first kind of victim of the American-style of politics that [former prime minister Stephen] Harper adopted in Canada, which is very much about destroy your opponent. He's not your opponent he's your enemy."

During the 2008 federal election, Harper's Conservatives portrayed Dion as a bumbling, wimpy intellectual whose proposed "Green Shift" carbon tax would spell economic disaster for Canada.

Dion pauses as he announces his plans after failing to defeat the Conservatives in the 2008 federal election. (Adrian Wyld/Canadian Press)

"I think he's been vindicated by the green plan as more and more countries, including Canada, move toward the adoption of a carbon tax," Donolo said.

The Liberal Party lost 19 seats in that election.

"If people are asking why, it's because I failed," Dion told reporters a week after his defeat. He said he would step down as leader at the party's next convention, scheduled for the following spring.

But that exit was hastened by a botched attempt to replace Harper's minority government with a coalition supported by the NDP and Bloc Québécois. Dion bowed out as leader in early 2009.

Yet instead of following the traditional path of defeated political leaders, Dion decided to stay on the Liberal team. Some called it stubbornness, others said it was pure loyalty.

Loyal Liberal soldier in opposition

Many other Liberals opted out while Ignatieff led the party. Far fewer hung on after Ignatieff's even more disastrous defeat in 2011. But Dion stuck it out, covering several bases in Parliament while also helping to rebuild a party left in ruins.

At that time, Rae was the party's interim leader.

"On unity and on climate change, he was strong. I mean, he was strong and he was forthright and he was clear," Rae said. "He provided real intellectual leadership on both those critical issues not only for Canada but for the world."

Liberal Leader Michael Ignatieff and Dion share the stage during a tribute to Dion at the Liberal leadership convention in Vancouver in 2009. (Adrian Wyld/Canadian Press)

When Dion believes in something, Jennings says, he sticks around and when things get tough he doesn't run away.

"He's got enough humility that he's prepared to do whatever the leader wants."

Sources have said Trudeau offered Dion the job of Canada's ambassador to the EU and Germany, but the prime minister didn't divulge specifics to reporters after the cabinet shuffle.

"I have offered him a very important, senior position that is something that is going to be key for me in the coming years as we move forward," Trudeau said.

Dion's public statement offers few clues about his future beyond him saying he plans to deploy his efforts outside politics.

"I have enjoyed political life, especially when I was able to make a difference to benefit my fellow citizens," he wrote, before adding a cheesy joke, "I emerge full of energy ... renewable!"