It's not a threat to you, it's a consequence of your husband's actions. He created a child, and is now badmouthing the mother of his child. Her child (and clearly it isn't their child from his perspective) deserves to be taken care of by the people that brought him life.

Whether or not she is smart, whether or not she is vengeful, he had a child with her, and is actively telling you that she's the bad person in this situation. He is. Fair and square, he is the absolute bad person. For you to stick by him is... моя мама сделала тоже самое. Очень глупо.

Make up your mind. Do you want to stay in the US at any cost? Or do you actually want to be with this shitty person?