The rest of the I international community will though, who could ever countenance entering into an agreement with the UK ever again (US included) if they are seen to be welching on an agreement already made?
Plus, the EU are the UK's closest and biggest trading partners, it is a good deal with them that would seem to be more important to finalise.
What has been said by the US is that they will not stand idly by if the Good Friday Agreement is allowed to go down the swanny by The UK, and the threat of violence returning to the six counties would not be good for anyone.