Need some advice I am trying to write a validator which only fires when a specific value in a dropdownlist is selected.

I have two drop down lists on this form one for Countries and the other for US States, the States dropdownlist only shows when the United States is selected from the Countries dropdownlist. 

I need a validator that makes the State dropdownlist list a required field only if the United States is selected as a country. 

As background information this a MVC3 Web Application and the show/hide code for the States dropdownlist is JQuery.