When Ed Miliband visited Afghanistan recently he was accompanied by shadow foreign secretary Douglas Alexander, shadow defence secretary Jim Murphy and ITN's Tom Bradby. Upon their return, Bradby declared that Jim Murphy could be a future leader of the Labour Party. He had gone out to Afghanistan to follow Ed, and came back enamoured with Jim. "The sand clearly got to his brain," Murphy chuckles.

On paper, Jim Murphy appears eye-rollingly careerist – a former NUS president, elected as Scotland's youngest MP and a factory-made New Labour disciple, who admits to being "spoiled" by 13 years of government.

But what distances Murphy from other identikit professional MP-bots is that he became a politician by accident. No-one expected him to win his seat in 1997. He became an MP before his time.

In his large parliamentary office, Murphy finishes making two cups of tea. "Look, I’m not getting involved in that," he tuts when asked about leadership ambitions. "Anyone who has a plan to make that happen should be disqualified from ever making it happen. You do what you do, and see how you get on," he says in his soft Glaswegian lilt. "We’ve got a boss, and I want him to be boss for 10 years or more. There’s no vacancy. And I don’t want there to be."

Murphy, though, is courting attention. In early March, he will deliver what someone close to the MP describes as a "step-changing" speech on Labour's approach to defence. It's bound to get him noticed.

"In politics you make a lot of speeches, but some of them you put more effort into than others." Murphy smiles. "The argument I'm keen to develop is, how do you stop one-and-a-half unpopular wars – with Iraq certainly being unpopular and Afghanistan at least partly there – creating an unpopular concept? The unpopular concept is that you have a responsibility beyond your own borders."

It's a difficult place for a Labour politician to go. After the messy politics of Iraq, humanitarian interventionism took a backseat to hand-wringing and humbled rhetoric. Ed Miliband endorsed the language of regret, declaring that Tony Blair's government was "wrong" to invade Iraq.

Former defence secretary Bob Ainsworth also distances himself from the invasion, despite having been a whip for the Commons vote at the time. "We are at the position now where it is almost beyond argument that Iraq was a mistake," Ainsworth says. "There were no WMDs and that's what a large part of the support in the House of Commons was about at the time. You've got to let your leader say what everybody else says and thinks."

"There will be a tendency to [turn away from interventionism]," he adds. "People will say that we got our hand caught in two prolonged counter-insurgency operations and we don't want to go there again."

But for a self-described "centrist", Murphy feels that the time has come to separate the emotion of Iraq from the need to protect national interest. "It's my business what happens beyond my front door," he explains, "but it is also our business what happens beyond our borders. I don't want to let the anger about Iraq trump the shame of Rwanda. We sat and watched what happened in Rwanda as an international community. Everyone said 'never again' after the previous genocide. How do you prevent people's genuine fury about Iraq stopping us from ever exercising force in the future without appearing like the 'more war' party?"

Is Murphy concerned that reiterating the case for humanitarian interventionism could be seen as undermining Ed Miliband's distancing stance on Iraq? " I didn’t want Ed Miliband to beat one David – David Miliband – but I sure as hell want him to beat the other David – David Cameron," states Murphy. "I just want to get Ed Miliband into Number 10. I’m in no doubt that Ed is connected to this agenda. He has respect for the armed forces and the responsibility that we have – but you’d have to ask him."

Murphy voted for the war in Iraq in his early days as an MP, although he admits that he would not have if he'd known there were no WMDs. "It is right that, after Iraq, we're pretty stringent about when we do it and when we don't. But you can't dig your head in the sand. Iraq teaches us that it's easier to bring down a government with military power than it is to build a country with military power. You invest in your armed forces to protect your national interest, and sometimes that means your armed forces having to serve internationally."

This sentiment echoes that of Tony Blair, speaking in Chicago in 2009 on global affairs, almost 10 years after his famous speech in the same city, concerning Kosovo: "I understand completely the fatigue with an interventionist foreign policy, especially when it involves military action that takes its toll on the nation's psyche," Blair said. "… We live in the era of interdependence; the idea that if we let a problem fester, it will be contained within its boundaries no longer applies."

Murphy agrees that the world is more complicated than when Blair delivered his first Chicago speech in 1999. "But a sense of that responsibility is still the same."

That sense of responsibility for Iraq is also a personal one. It was also the first British political issue that Murphy was involved in. "I organised a demonstration. It was the first anniversary of Halabja in 1989 [where chemical weapons were used by the Iraqi government on the Kurdish town]. I had some Kurdish friends, and we organised a demo in St George's Square, Glasgow. Fifty people turned up, no journalists. No-one was interested. So the Iraq thing was something that I was always personally committed to."

Brought up in Glasgow, Murphy lived in a flat with four generations of his family: "Me, mum, dad, my brother, my granny and my great-granny. My bed was the drawer. I got the bottom drawer, and my brother got the top drawer. My politics are born of being brought up poor in Glasgow, and then white in South Africa."

Aged 12, he emigrated with his family to Cape Town, after his father became unemployed. "In South Africa you get a different type of inequality. Not based on class or family structure, but on the obvious thing, race. I went to a white-only uni. I went to a white-only school. I played football in white-only football teams. The only Africans you would meet would be folk who worked in the school."

He credits these two experiences with his politics of "utter, unconditional intolerance of prejudice – it’s in my DNA, having seen what the politics of discrimination can do."

When he was a younger politician, Murphy admits to lacking this confidence of conviction. "As a kid, I grew up one street outside my constituency. I’m a bit angry with myself that I didn’t have the self-confidence to say so during the 1997 and 2001 elections. I thought that these good folk here maybe don’t want someone like me being their MP, because politics is partly about aspiration. I embarrassingly thought that these folk wouldn’t want to trade down for a guy from a housing scheme."

He bristles: "This is like This is Your Life. I'm not into all the emotional stuff. I’m very much into the politics of emotion and nostalgia, but less into talking about myself. Let's go back to the defence stuff. That'll be easier."

So we return to defence, although it's not necessarily easier. If Iraq was complicated, Murphy believes that the situation in Afghanistan is more complicated still. He explains there are at least four Talibans operating in the country. "Rebuilding a country is much more difficult than toppling a dictator," he says. "There isn’t going to be a peace deal." Having recently returned from his first visit to Afghanistan, Murphy is clearly affected by the experience. "I don't want to talk about it, but I was taken aback by the brutality of the regime, by people that would do that deliberately." He is now calling for more training of the Afghan forces. "We're trying to grow an army of over 300,000 people, and most Afghan recruits can't count the number of bullets to go into a rifle."

Ainsworth believes that an "enormous rethink" is needed off the back of Afghanistan. He is calling for an inquiry that would ask "any and every" question on the invasion. "We're all waiting with baited breath to see what conclusions Chilcot comes to," he says, "but there has to be a similar process on Afghanistan. There are similar issues that need to be thought about – not least the military doctrine on counter-insurgency. There has to be a full public inquiry, in all probability on Afghanistan, sometime in the future when the troops come back."

For Murphy, Afghanistan should not be the last intervention beyond national borders. "If Kosovo was to happen in 2017, so we’re out of Afghanistan, I don’t want to get into a position where we would say, post-Iraq, post-Afghanistan, 'we couldn’t do another Kosovo'. It's important to make that argument. I’m not trying to nudge things in favour of another military intervention anywhere, but you shouldn’t let the residual, real anger that there is about Iraq defeat the pride that we have in what we did in Kosovo."

The big question about this attempted shift in approach is, why now? Much of the answer lies, surprisingly, with the electorate. "I want to end the view that Labour is the party of the NHS, and the Tories the party of the armed forces. We need to show that Labour's commitment to the armed forces on any day is as important as the commitment to the NHS."

"It’s about how the Labour Party looks more committed to the armed forces in terms of its appearance, how we sound and how we reconcile ourselves to our record." The selection of former soldier Dan Jarvis as the Labour candidate for Barnsley Central is just one example of this. "We don't have enough MPs or councillors who have experience of the armed forces," says Murphy. "Then Major Dan comes along… I'm cock-a-hoop about it."

However, Murphy lacks the same chirpy confidence when it comes to Labour's record on defence spending. The Conservatives have criticised him repeatedly for lacking a credible economic plan. Liam Fox's camp condemns Murphy's opposition to cuts without explaining how he would make savings. "There are many areas where Murphy is opportunistic, and criticises cuts when he should be supporting tough measures to sort out the MoD deficit," said one CCHQ-ite.

On Labour's financial record, Murphy freely admits that they "got some things wrong". "Like all governments, we didn't get on top of defence procurement," he says. "The big money-saver we're looking at is how to do real-cost procurement differently – where a company says, 'This is how much we can do it for', and the government says, 'This is how much you will do it for'. Not adding £1bn because you've discovered a complication. That could save a lot of money."

But Murphy is vague on other potential savings: "We didn't have an SDSR of our own, so I'm not today going to have announced what it would have been." It is undoubtedly his weakest answer.

Outside of defence, Murphy spends his efforts on his seat. Legend tells that he has had tea with every person in his constituency. "Most people," Murphy corrects. This is because he is not the natural incumbent for the seat; previously, East Renfrewshire was the safest Conservative constituency in Scotland. Before 1997, the Labour Party had not won there since the 1920s. "On election night, I did not think for a second I was going to win," he recalls. "It’s not false modesty, or a ‘gee, shucks’ thing, it’s the truth."

Three elections later, he describes his majority of 10,420 as "decent", but is not complacent. Throughout the interview, he checks his language, concerned that the wrong sentence could end up on "a Tory opponent's leaflet". Does he regret that his election happened so early? Murphy pauses. "Yeah, I do," he admits. "I didn’t try to get it, I wasn’t expecting it."

"It took me three years to adjust to being in Parliament. Here are these guys – Tony Blair and Gordon Brown – that I’d seen on telly. I didn’t come here with someone who was going to look after me. It took me about three years to have a proper conversation with either Blair or Brown."

"You can hide here," he says, gesturing around his parliamentary office. "My predecessor, a Tory, a decent man, got himself into trouble, in the loneliness of the place, through alcohol. He was a good person, but it can be a very lonely place."

But Murphy is not lonely in Westminster. Since David Miliband lost the Labour leadership, the right of the party is looking for a replacement to counter Ed Balls. Tom Bradby is not the first to notice Murphy's potential. He is popular, confident, and building a strong team. Redefining Labour's stance on intervention could prove a game-changing moment for him. 

It will be tricky, though. He's asking a cautious leadership to re-embrace interventionism after being burnt by Iraq and Afghanistan. He's also asking the hesitant armed forces, who were nurtured by Margaret Thatcher in the 1980s, to place their vote with Labour."You do what you do, and see how you get on," he says.

He may have become an MP before his time, but Jim Murphy is not going to sit on his hands and wait for a return to government. Watch this space.

Tags: Afghanistan, Bob ainsworth, David Miliband, Douglas alexander, Ed Balls, Ed Miliband, Iraq, Jim Murphy, Tom Bradby, Tony Blair