I think it's a lot more nature than nurture – what an individual dog is born with; but no doubt at all that dogs that serve in specific protection and combat roles like that have been extensively trained, desensitized and tested. Their capabilities have been nurtured and increased.
A dog, however, either has the goods to be worked with or it does not.
You might be able to get a mediocre dog trained and nurtured up into satisfactorily doing one of those jobs, but it's not a dog I'd want to place my trust in.
I also think of dog sports. I used to put a ton of stock in Schutzhund (IPO / IGP) titles even though I knew it is mostly or fully a game to the dogs. These days I'm less impressed. I know you can spoon-feed a dog to titles. I feel, quite frankly, like my Oji's dam, an IPO III, is an example. It was shared with my by his breeder when I asked about Oji's trouble achieving an effective full-mouth bite that his dam had a lacking bite when younger and it had to be trained and brought up to snuff. By contrast, my previous Dobe had a hard, effective full-mouth bite with zero training and at a very young age.
My mind goes to the footage I saw years ago of a Secret Service dog (German Shepherd or Mal) that was sent toward a man that had jumped the White House fence. Just as the dog reached the man, he landed an effective hard kick to the dog's head. The poor dog was stunned and turned and ran back toward the handler. I mean, I guess you could say that if a dog, or other animal has never been exposed to anything like that, it is understandable that it would retreat from such a shocking, unfamiliar experience; but I'd expect a dog that I would describe as "fearless" to respond by staying engaged and going in for the kill. I mean, it's win, or die trying. There's no retreat.
So, to answer your question (with my opinion

) I would say that fearlessness, or near fearlessness, is a naturally-occurring trait in an animal. I think it's vastly more prevalent in wild animals than in our domestic dogs.