In the Game of Thrones Season 7 finale, Cersei Lannister surprisingly agreed to a truce with Daenerys Targaryen: they would cease fighting while the White Walkers were dealt with, then deal with everything else once that threat was taken care of. Cersei's only had one deal-breaker: she didn't want Jon to choose sides between the two queens. But Jon refused Cersei's truce on Game of Thrones, so now everyone is at each other's throats again.
All Jon had to do was reassure Cersei that he totally wasn't going to take sides, even if he was still secretly supporting Dany in his heart of hearts. One little white lie would have gotten the Lannister army off their backs while they dealt with the White Walkers, giving Dany and her people time to handle all of their pressing concerns. Unfortunately, Jon just couldn't do it. He might not be Ned Stark's biological son, but Ned's lessons left an impression on him all the same. He wasn't willing to make a truce if it was based on a lie.
Tyrion definitely thought Jon made the wrong call — and fans did too. But it turned out Jon's good intentions didn't actually ruin everything, even if it seemed like it at first.
When Jon initially refused Cersei's truce, it seemed like it was game over. All of the work arranging the meeting between the three rulers was for naught. Cersei stormed out with her entourage in tow, fully set on her decision; despite the threat of the White Walkers, she would not pull back her armies. She would let Jon and Dany to handle things on their own — maybe they'd even be destroyed, leaving Cersei the final ruler by default. But then Tyrion took matters into his own hands.
After a single conversation with Tyrion, Cersei was ready not only to cease her attacks but to join forces with Jon and Daenerys. She did her own truce one better by supplying them with more soldiers to fight the battle against the White Walkers. It looked like even making the wrong decision in the moment turned out to be the right one for Jon in the long run: everyone just had to wait a little while and leave cleaning up the mess to Tyrion. Now they had everything they needed, and Jon's reputation for honesty remained spotless.
Or so it seemed, anyway. As it turned out, the situation was really too good to be true. Cersei was doing what Jon couldn't: lying so that she could get what she actually wanted in the end. She only pretended to follow through with the truce so that she could plot behind Jon and Dany's backs. Maybe Jon should have been a little less honest in that first meeting, but it's always a dangerous proposition to make a deal with a Lannister. The Season 7 finale proved that.