It's hard to imagine that Prince William and Kate Middleton as anything other than a happily married couple and parents to three charming kids. However, do you remember that once upon a time, a lot of people suspected they wouldn't last? 

They dated for nearly a decade before getting married, so it's hard to blame people for thinking that they won't take it to the next level

As it turns out, thoug, Wil and Kate are more devoted than we all knew. 

Royal expert and biographer Katie Nicholl wrote a book called "Kate: The Future Queen." In the book, she revealed that the couple made a specific "promise" once they rekindled their romance after breaking up in 2007

The reason for their breakup was that the now-Duchess of Cambridge wanted security about their future, but the future King of England wasn't ready to get married just yet. 

The couple jetted to the island of Desroches -- in Seychelles off East Africa -- to spend some quality time together. They went there to figure out what their future together was going to be. 

According to Nicholl, they made a pact about their long-term future together. 

"There on the paradise island, William promised Kate he was in the relationship for the long term," Nicholl revealed. "With the ocean beneath the sky, they made a pact to marry."

The book explained that Prince William promised Kate Middleton a diamond ring, intending to "take the pressure off and just promise we'll end up together."

According to a source close to the couple, they compromised and agreed to wait until Prince William felt ready.  They didn't decide to get married there and then; what they made was a promise. A pact.  

"William told Kate she was the one, but he was not ready to get married. He promised her his commitment and said he would not let her down, and she, in turn, agreed to wait for him," Nicholl furthered.

The Rest is History

Prince William proposed to Kate Middleton in Kenya in 2010 and later announced in the same year that they were engaged.

In April of 2011, the duo finally tied the knot and officially became the "Duke and Duchess of Cambridge." They later have their children of their own: Prince George, Princess Charlotte and Prince Louis. 

During their engagement interview, Kate and William talked about their temporary split, telling the interviewer that the separation was a vital part of the "development and strengthening of the relationship." 

Speaking to Tom Bradby of ITV, Prince William said that they were both so young, and it was at university where they found themselves while being different characters.