Why did Voldemort want to kill the Potters in the first place?
This may well be because Lily and James were some of Dumbledore's most powerful and avid supporters as well as integral members of the Order of the Phoenix. So the biggest threat to V-Voldemort [get a grip guys, it's just a name!] and his Order.
But why go through all the motions of murdering an entire family, even though they are cooped up in a hidden house and posing no immediate danger?
References preferred. But justified conjecture is also welcome.
Edit: I wrote this just after reading that Neville broke the prophecy. So I thought it might never be answered, which is why I asked the question. I now know the full reason.
I just wanted to point out the irony: the parents of Neville were Aurors and had defied Voldemort thrice as well. Now guess at the end of which month Neville was born: July. By going after Harry, Voldemort fulfilled the Prophecy with Harry as adversary, but Neville would have been in Harry's place had Voldemort tried (and failed) to kill Neville.
There was this prophecy:
Sybill Trelawney's first prophecy, sometimes simply known as The Prophecy, refers to the prophecy made by Sybill Trelawney to Albus Dumbledore at the Hog's Head Pub in 1980. It referred to a boy born at the end of July, who has the power to defeat Lord Voldemort (which Voldemort took to mean Harry Potter) and Lord Voldemort himself.
In what proved to be a vain attempt to circumvent a prophecy that a boy born at the end of July of 1980 could be able to defeat him, Voldemort attempted to murder him when he was a year and three months old, shortly after murdering Harry's parents as they tried to protect him.
Voldemort feared that Harry was the one the prophecy was talking about, since he was born at the end of July. That's why he wanted to kill him. Harry's parents were 'just in the way'. Voldemort killed them because they were trying to protect Harry
Here is the exact prophecy:
The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies ... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not ... and either must die at the hand of the other for neither can live while the other survives ... the one with the power to vanquish the Dark Lord will be born as the seventh month dies ...
While your answer is good by the "plot" of the complete series, I digress nevertheless. It was while reading book #4 where it says that the quiddich cup was so heavy that 4 (or 6?) wizards had to carry it when I realized that Rowling had not only hired a ghostwriter to continue her series, but one who didn't even care to acquaint himself with the story. Wizards do every crap with magic from washing dishes and stirring their coffee to making fire blue, simply because they can -- but they don't think of levitating a heavy object on a wizard-only event full or wizards doing wizard stuff. (...)
**Therefore:** The prophecy, which appears only in the later books (which contain many similarly striking examples of character and story errors), cannot be seen as canonical. It was said in the first book that Voldemort was after the Potters (parents). From this, we have to assume the boy was just attacked by "chance", because he was there. And, of course, because that starts the story.
@Damon, if it's released by JKR or stated by JKR, it's canon. It may not be your personal head-canon, but that doesn't change its official status. Even if your theory of a ghostwriter is correct, JKR would have likely provided the outline of all plot points and major details to the writer. Alan Rickman has said in interviews that he was given small bits of information about Snape long before they were revealed in books.
Don't forget that the Prophecy could have also pointed to Neville Longbottom (Don't remember the exact quote), it could be speculated that because of the Potter's standing in the Order of the Phoenix, Voldemort would naturally assume that their child would be as detrimental to him if not more so than his parents.
The complete prophecy indicates that Voldemort must 'mark him as equal'. By attempting to kill Harry, Voldemort 'double-binds' himself and Harry to it. Without this final act, the prophecy could apply to any boy born in July, whose parents were in the resistance (eg. Neville Longbottom).
Like any author, JK did not necessarily have the entirety of every plot point fully fleshed out when writing the first of 7 books... But beyond that, the books are almost entirely _Harry’s_ perspective and what people around him know and/or are willing to share with him — specifically compounded by the fact that the full prophecy was known only to Dumbledore, everyone else’s opinion was not fully informed. It’s a bit of a reach to reject _plot development_ by reading something into statements that was never there, let alone manufacture a ‘ghost writer’.