function showStateForUSOnly() { 
  var selectVal = $('#ddlCountry-div select option:selected').val(),
	    action    = (selectVal == 'United States') ? 'show' : 'hide';
	$('#ddlState-div')[action]();
}

$(document).ready(function() {
  showStateForUSOnly();
	$('#ddlCountry-div select').change(showStateForUSOnly);
});