Nato air strikes were intensifying in Libya, a new television show called Game Of Thrones had just premiered - and Prince William finally made a young lady named Catherine Middleton his bride.

More than two billion people watched the wedding at Westminster Abbey on April 29, 2011, an event billed as the biggest royal event since ‘Charles and Di’.

And today, the Prince and Princess of Wales will celebrate their 15th anniversary.

Aides are discussing how to mark the event publicly, given that it lands midway through the King and Queen’s high-profile visit to the US. Privately, however, friends and associates say that William and Catherine have very much ‘found their groove’.

Jamie Lowther-Pinkerton, a former private secretary to the couple and godfather to Prince George, is better placed than many to know what makes them tick.

He first met the future princess when he joined William at St Andrews for his graduation in the Spring of 2005, describing them as ‘incredibly well-suited. Even on first meeting them, that was obvious’.

They were already a couple by then but unwilling to offer up their relationship for public scrutiny.

Many talked about William’s vocal determination to protect his girlfriend and future wife, particularly given his late mother’s tragic history, but few realise that this was very much a two-way street.

I would go so far as to say that the princess’s own strength - equal to William’s but quieter - has been severely underestimated over the years, and I believe it has been crucial.

‘It was, and is, a meeting of minds and equals between the two of them,’ says Lowther-Pinkerton. ‘The idea of him wanting to protect her is a nice image, but it goes both ways. She is very protective of him too. There’s always been a quiet confidence about her.’

It was that unshowy self-belief and grounding that drew William back to Catherine after briefly calling time on their relationship in 2007. Friends say the reunion also had much do with the strength of Catherine’s close family background.

There’s no doubt in Lowther-Pinkerton’s mind that, while she is an ‘absolute rock, very intelligent and intuitive and strong’, the fact that she comes from such a tight-knit family - parents Michael and Carole Middleton still live within a short drive of all their three children in Berkshire - was hugely attractive to William.

The former private secretary recalls: ‘My favourite image of the entire wedding was Mike [Middleton] sitting on his lawnmower the next day. That, to me, summed up the whole family set-up. Having walked his daughter down the aisle in front of billions, this rather self-effacing man was, the very next day, back in his garden. To me, that image of Mike represents the very essence of being English.

‘Through inevitable force of circumstance, William hadn’t always had that sort of family background himself growing up. And it’s quite clear that Catherine’s happy family upbringing has been reflected in the way they are raising their own children.’

Many within the royal household refer to Catherine as ‘the Steel Marshmallow Mark II’, a play on the late Queen Mother’s nickname: her soft exterior belying an indestructible core.

And while there was certainly a little envy of this new star turn within the often-territorial royal household after she first arrived, Catherine was very much helped by the late Duke of Edinburgh, who took her under his wing.

‘I know people think of him as a bit gruff but, when she joined the family, he was incredibly kind and welcoming and singled her out for attention with some very thoughtful gestures and advice on what charitable work she could pursue,’ says a source.

‘She had lost her own grandparents, so it became a very important relationship to her. They frequently wrote letters to each other.’

In public it is William, 43, who often takes the lead. But it is very much an equal partnership, and William deeply respects his wife’s judgment.

‘They make all the important decisions together and support each other’s work,’ says a friend. ‘You couldn’t find two people closer. The difference is they just don’t feel they need to show it for the cameras. They aren’t performative.

‘One of the things that the princess, in particular, has always been so impressively clear about is that she is allergic to being given advice about how things will come across, or look through a PR lens. That’s the total antithesis of how she operates.’

They have come through a tumultuous few years, with the implosion of William’s relationship with his brother, Prince Harry, and Catherine’s cancer diagnosis.

‘From losing his grandmother, to all of the awfulness of the princess’s health, as well as that of his father, it’s been a tough period for William,’ a source says.

Notably, Harry isn’t mentioned once. I’m told there is sadness, naturally, at how things have turned out, but William has very much placed the issue of his estranged brother to one side, to ‘focus on the things that matter’.

His uncle, Andrew Mountbatten-Windsor, is more of an immediate headache, the ramifications of which may last well into William’s own reign.

I am told suggestions the heir to the throne was a prime mover in his banishment to Norfolk are untrue.

While despairing of his uncle’s behaviour, the future king was deeply concerned about Andrew’s mental health.

As for reports he has ‘frozen out’ his cousins Princesses Beatrice and Eugenie, a friend says firmly: ‘It’s complex, but he’s kinder than that. Easter [when it was claimed that William only agreed to attend if the sisters weren’t there] was absolutely nothing to do with him.’

Moving their family to the isolation of Forest Lodge on the Windsor estate has been crucial to the Waleses’ own sense of family harmony.

‘It’s given them a new lease of life. It’s helping them to put the unhappy memories of the past few years behind them,’ a source explains.

‘The move has given them a renewed sense of purpose and vigour. They are happier, and you can see that visibly. Adelaide Cottage [their former home] coincided with a really unhappy time in their lives and they were ready to start a new chapter.’

Another source says: ‘They have become more comfortable with the increasing state duties they take on but, at the same time, like nothing more than sitting at home and watching Traitors [a recent obsession, apparently] three nights a week.’

The next few months will see another sea of change in their lives with their eldest child, Prince George, who turns 13 in July, heading off to boarding school.

‘Like all parents they are both equally thrilled for him and dreading it, I think,’ says a friend.

The princess hasn’t undertaken a working foreign visit since before her cancer diagnosis, but aides haven’t ruled out her taking up the baton again this year.

William is also continuing to work on his long-term master plan to promote his work with, as he has described it, ‘maybe a smaller "R" in the royal’.

‘A big part of that is being open with how he feels on things, such as the work he has done on mental health and suicide prevention,’ a source explains.

‘When thinking about the future he himself has said change is on the agenda. That is true. But it’s worth noting that both he and Catherine are also very much “conservative with a small c”. They both respect the tradition that comes with their roles,’ says a source close to them.

They add: ‘Will he get rid of garden parties and Trooping the Colour? No, of course not. But might those things feel a little different in the reign of King William? Yes. They both want things to feel more relevant and “of today”, whenever that might be.’

Another source says: ‘When you want to make change, you have to make really difficult decisions. He’s prepared to shake a few trees but not to cut down the whole orchard.’

And the princess is by no means a passive participant. ‘When the time comes [for them to step up as King and Queen], they will enter into the roles in the same way they have approached the first 15 years of their marriage. As a team.’