No one is the queen of the broken promise quite like Cersei Lannister. So you'd be right to be suspicious about Cersei's promise to marry Euron Greyjoy on Game of Thrones. It seems awfully convenient that she is waiting until after the war to wed him, but Cersei has surprised us before. Maybe she's being genuine about this pledge?
Euron first proposed to Cersei in Season 7, Episode 1. He sailed to King's Landing and fed her some story about how "ever since I was a little boy I wanted to grow up and marry the most beautiful woman in the world. So here I am with 1,000 ships and two good hands." The dig at Jaime aside, Cersei didn't trust Euron, who regularly turns on allies and murdered his own brother. She told him she would not marry him.
But Euron wasn't giving up that easily. He promised to return with a gift for Cersei that would show how serious he was. When he came back in Season 7, Episode 3, he gifted her the captive Ellaria Sand and her sand snake daughter as punishment for the time Ellaria killed Cersei's daughter. Cersei was impressed and gave in to his request. "You shall have what your heart desires when the war is won," she said.
Holding off on marrying Euron until after the war is a smart move. For one, Euron could die before then. Or, if Cersei loses the war, she can re-evaluate and choose a different person to marry who will get her more power again. Everything Cersei does is in the name of power. If marrying Euron will bring that, she'll likely go through with it. But since she has the promise of his army's support even without marrying him right now, there's not a whole lot of an incentive for her to marry him after the war. She never remarried after Robert Baratheon died, but perhaps she will now to legitimize her incest pregnancy, or maybe to keep Euron from rebelling against potential rejection and killing her. Cersei is smart, and she'll do what's best for her — whatever that might be.
As for Euron, he seems to know she won't necessarily keep her word. He knows she and Jaime have had a romantic relationship, and Euron likely understands that could come between his goal to be married to Cersei. So it seems he's going to do his best to get what he wants now, while he waits for the war to end. He already convinced Cersei to let him into her bed chamber, and he wants to get her pregnant — which she may already be unbeknownst to him, although he'll certainly take credit for it. So he's coming out on top for now, at least.
According to the actor who plays Euron, his character really does want to go through with the wedding for more than just political reasons. "He likes her. He’s intrigued by her," Pilou Asbæk told Making Game of Thrones. "Cersei’s certainly beautiful and she has power, and those are two very sexy things. So he’s genuinely attracted to her ... He’s intimidating and evil, and in many ways a menace, but for me it was very important it was genuine love."
Cersei's feelings may not be genuine love, but she does seem amused by Euron's confident nature. And with Jaime abandoning his sister/lover to help Jon Snow, Cersei may want to make Jaime jealous by going through with her wedding.
It would be fitting for a show that ruined weddings (the Red Wedding, anyone?) to conclude with a wedding. But fans will just have to wait until the war ends to see if Cersei keeps her promise.