Game of Thrones spoilers below, duh.
After Game of Thrones episode seven, "The Broken Man," a Photoshop master deciphered Sansa's secret letter, confirming its fairly obvious destination and message: "Littlefinger" and "help me with your big army," respectively.
Now, the robust subreddit r/Gameofthrones has another opinion on the ongoing chain mail saga in the North. From a user named ajxnjd:
This is about Sansa's letter to LF and why she sent it.
I think I cracked this up. Remember when Sansa insisted in sending Brienne and not sending a raven to ask her uncle for help? Her argument was that she didn't want Ramsey to intercept the Raven and knows about this. But now it seems like she's gonna send a raven to ask little finget for help. But why? Shouldn't she be afraid that Ramsay will find out? Well here's my theory. Sansa wants Ramsay to find out about little finger army. Ramsay, being the mad dog that he is, will send a good part of his forces to deal with LF and knights of the vale. Now, Sansa and Co will fight a reduced Ramsay army due to this and they win easily.
TL_DR: Sansa will screw LF over to win Winterfell.