Your best first step would be obtaining a custody order that stipulated your child could not be removed from the jurisdiction without prior written permission from both parents.
When a parent wrongfully removes a child, the other parent has a number of legal solutions to protecting their right to custody. There is a cause of action for abduction as an interference with custodial rights.
The most important law in international child abduction is the Hague Convention on the Civil Aspects of International Child Abduction. This agreement deals with the return of the child and the enforcement of other custodial rights. Your would need to seek the assistance of a lawyer to file with the central authority, which is the State Department