Responses
Reading veteran here, I'mma call THIS as the bullshit resolution: They live together for a while, she catches feels for him or he for her and when it's revealed nobody will give a shit 'cause love and all that jazz. XD
Pretty sure this is the cliche they will go with.
His army already invaded/ ransacked their country and took it over. If they have a child together, (even if there's a slim chance that she's not actually related to the princess, but knowingthis trope she'sprobably a bastard child of the King) because she shares a similar appearance to the princess, the child could be be used to claim regency/sovereignty over the kingdom. (づ￣ ³￣)づWho could refute such a claim, if all her closest aides are dead?? And when he finds out she's not the ACTUAL princess!! Her life and anyone one else's are no longer of value, then he could just burn the whole kingdom to ashes...maybe I'm just reading too much into this... ╮(￣▽￣)╭