So far, Season 7 of Game of Thrones has been a season of callbacks. Events and people that seemed buried in previous seasons are coming back out of the shadows. Jaime learned the true identity of Joffrey's killer. Nymeria the direwolf came back, if only briefly. Plus, of course, the return of the much-beloved Gendry (I still can't get over that, by the way). There have also been some more cryptic references to past seasons — like the letter Arya found in Littlefinger's room on Game of Thrones. So what did that little scroll say, exactly? It goes all the way back to Season 1.
If it's difficult to remember that scene amid all the epic-ness that was last Sunday's episode, "Eastwatch," let me remind you: after following Littlefinger around in what she assumed to be a stealthy manner, Arya watched him receive a raven scroll, ostensibly on Sansa's behalf, which he then hid in his room. Arya broke in and found the scroll, which turned out to be the one that Sansa had been forced to write to Robb after Ned was charged with treason, asking that House Stark swear fealty to King Joffrey. As she left Littlefinger's room, the camera showed Littlefinger watching her, revealing that he had been onto her the whole time.
So, what was Littlefinger's plan here? If you haven't been watching the weekly post-episode interviews with David Benioff and Dan Weiss, now's probably a good time to start. As Weiss explains, Littlefinger is "looking for a way to prevent this sister bond from developing further." The closer Arya and Sansa become, the less power he has over Sansa, and the more likely it is that he'll eventually be ousted. The letter that Sansa wrote back in Season 1 is the perfect thing to drive a wedge between the two Stark girls, who have never exactly been the best of friends. Arya never knew about that letter, which doesn't cast Sansa in the best light, even if she wrote it under duress.
It's clear from Sansa's expression as she watched Arya and Brienne fight in the preceding episode, "The Spoils of War," that she isn't incredibly thrilled with the path her sister has taken. Which is fair — as awesome as Arya is no one wants to see their little sister grow up to be a cold-blooded killer. Littlefinger, noticed this and is (as usual) five steps ahead of everyone else, knowing that Arya will most likely have a bad reaction to finding that letter and this will only make Sansa more doubtful of her sister. "[Littlefinger] knows full well that with what's going on in Sansa's head with regards to Arya," Weiss explained. "When Arya starts to act dangerous and act angry, he knows where Sansa's going to turn." To himself, of course.
It looks like Arya's just going to have to learn the hard way that Littlefinger is not easily outsmarted. He's lasted this long, despite what most of the other characters probably want, and he's in it for the long haul. Arya may have finally met her match.