Cat1nthehat listen to her - don't just expect her to be receptive. Don't underestimate how very difficult having your world contract down to your nuclear family is at 13 - the age at which peers become more important influences than nuclear family to a young person, as is right and proper to their development.
Don't forget how stressful just being 13 is. Everything is constantly changing, not least your own body, your idea of who you are, your relationship with your parents and your realisation that they are as fallible as you are, your new interest in the opposite sex (or the same sex in new ways), in your appearance - the cataclysmic insecurity alternating with arrogance born of defensiveness that comes with that...
Then rip the ground out and say we're moving thousands of miles away from your friends, possibly extended family, possibly hobbies or sports integral to your sense of who you are, certainly the school system you know. Remember that American high schools are mostly portraid in the media as very cliquey and socially brutal places if you don't fit in...
You need to listen to her worries and let her know that you take them seriously and understand what an enormous deal this is for her.
Whether you ultimately move is a different question - it's not necessarily moving that's awful so much as being told that your feelings about moving don't matter and your problems are trivial compared to mum and dad having a long distance marriage. Don't expect the receptiveness to be one way. You listen to her... Then later (the next day or after s break for lunch/ snacks/ activities) you talk ...
Encourage questions, do you know anything about schools in the place you'd move to? Would she go to an international school or an ordinary American state school?