The official Game of Thrones Twitter account has just unveiled what was in Jon’s latest letter to Sansa in season 7.
Cersei Lannister has pledged her forces to our cause, as has Daenerys Targaryen. And if we survive this war, I have pledged our forces to Daenerys at the rightful Queen of the Seven Kingdoms. We are both coming to organise the defence of the realm.
Jon Snow. Warden of the North.
— Game Of Thrones (@GameOfThrones) August 29, 2017
Source: HBO CBR