Man, this is crazy. Can't find the exact post from a year or so ago (maybe it was another thread?) but we had a big discussion about the Others vs. R'hlorr and who was actually the "good guy." And from there it kind of dovetailed into a discussion of the Citadel and Faceless Men and what "allegiances" these organizations likely had (because let's be serious, you don't just have two super powerful organizations that are 100% neutral.
I don't do as much reading on theories and such as some of you guys... so please correct me if any of this is out of date or wrong. But since that discussion, GRRM has basically come out and given explicit quotes that do more than just imply that the Others are not "bad guys." Then you have last night's episode and the theory IrishLion just posted.
One thing I postulated on awhile ago was that the Wall was raised by the Others and not man... because duh. You don't simply engineer some giant fucking ice wall in the middle of war. However, a race of magical beings who are an embodiment of everything cold could rather plausibly raise a wall in either retreat or as part of a treaty.
You consider the history of races basically saying "men = scourge" and trying to stomp them out/cut them off. The Children of the Forest crushed the Arm of Dorne and then tried to crush the neck. Some time after all that failed and the Children made a peace with the humans... and then following that, the Others came to wipe shit out.
Did the Others already have a peace and line of demarcation set up with the Children set up way before the humans showed up? I'd say yes. They probably coexisted being all magical and stuff with the Children knowing their weaknesses, etc. To me, this seems to suggest that the Others were sitting around watching stuff go to hell because of uncaring/ruthless/stupid humans and in a self-preservation move decided to move south and handle their business. At this point, you get the Long Night.
So the Last Hero seeks out the Children... and somehow he gets them to help. Maybe they view the humans as less of a threat as the Others were their enemies first... maybe it goes all the way back to the Pact... I dunno. But they give the humans the ammunition they need, and then...
So now we get to what IrishLion posted. I've always surmised the above as the likely order of events, and nothing that has happened in the show has any impact on that, but the Night's King reveal and such throws a wrinkle in here. I had always assumed that with it being a fair fight with the obsidian and such that the Others said "fuck this noise", ran home, threw up a barrier, and called it a day saying "we'll be safe and sound now."
Lion's theory says it went more like 'let's make a deal...' and that the wall was the DMZ and that it was sealed with marriage. Personally, I don't know if I really buy all of that theory, but it's definitely a lot to think about.
I still think that the Targ conquest basically set things in motion... the Others saw the dragons (remember, one Targ road a dragon all the way up to the wall) and was like "oh shit, Iran got nukes." At which point they go from DEFCON 4 to DEFCON 2. They start gathering forces, building strength, and keeping a watchful eye on everything that is going on. I think the Citadel... which is all about history and shit... understood that dragons were going to fuck up the equilibrium and almost immediately started working on some angle to bring them down. And they succeeded, delaying momentarily what's happening now.
The Others held off because the threat seemed to dwindle away. And because they're led by the Night's King, who is either the original Night's King (i.e. the human who married an Other and "gave his soul to her" possibly becoming a hybrid of sorts that we see as a White Walker), or the progeny of the original Night's King and the Other. Either way... Stark blood, and he was probably OK with keeping his finger off the trigger until dragon people started burning Starks alive. I totally buy that part of the theory and it's kind of the missing piece on why they started mobilizing and actually pushing south.
I, personally, think that if you buy the Night's Watch commanders all married Others then it actually went down something like when it was time to move on, they "retired" by marrying an Other and combing souls or something. Maybe the difference is that the Night's King said "eff that, I want to rule this place eternally" and as such broke the agreement and started that incident.
As an aside, I also firmly believe that White Walkers are not pure Others. The Others that some (like Tormund) refer to as mist or pure cold seem like ethereal beings... whereas White Walkers as we've seen them seem like maybe a human/Other hybrid.