HTML Code:
<html>
<head>
<script type="text/javascript">
function showState()
{
document.getElementById('st').style.visibility = 'visible';
document.getElementById('state').style.visibility = 'visible';
}
</script>
</head>
<body>
<form name="form1" method="post" action="">
Select a country <select id="country" name="country" onChange="if(this.value == 'United States of America') showState();">
<option value="Afganistan">Afghanistan</option>
<option value="Albania">Albania</option>
<option value="Algeria">Algeria</option>
<option value="Angola">Angola</option>
<option value="Anguilla">Anguilla</option>
<option value="Antigua & Barbuda">Antigua & Barbuda</option>
<option value="Argentina">Argentina</option>
<option value="Armenia">Armenia</option>
<option value="Australia">Australia</option>
<option value="Austria">Austria</option>
<option value="Bangladesh">Bangladesh</option>
<option value="Belgium">Belgium</option>
<option value="Bhutan">Bhutan</option>
<option value="Bolivia">Bolivia</option>
<option value="Bonaire">Bonaire</option>
<option value="Bosnia & Herzegovina">Bosnia & Herzegovina</option>
<option value="Botswana">Botswana</option>
<option value="Brazil">Brazil</option>
<option value="British Indian Ocean Ter">British Indian Ocean Ter</option>
<option value="Brunei">Brunei</option>
<option value="Bulgaria">Bulgaria</option>
<option value="Burkina Faso">Burkina Faso</option>
<option value="Burundi">Burundi</option>
<option value="Cambodia">Cambodia</option>
<option value="Cameroon">Cameroon</option>
<option value="Canada">Canada</option>
<option value="Canary Islands">Canary Islands</option>
<option value="Cape Verde">Cape Verde</option>
<option value="Cayman Islands">Cayman Islands</option>
<option value="Central African Republic">Central African Republic</option>
<option value="Chad">Chad</option>
<option value="Channel Islands">Channel Islands</option>
<option value="Chile">Chile</option>
<option value="China">China</option>
<option value="Christmas Island">Christmas Island</option>
<option value="Cocos Island">Cocos Island</option>
<option value="Columbia">Columbia</option>
<option value="Comoros">Comoros</option>
<option value="Congo">Congo</option>
<option value="Cook Islands">Cook Islands</option>
<option value="Costa Rica">Costa Rica</option>
<option value="Cote DIvoire">Cote D'Ivoire</option>
<option value="Croatia">Croatia</option>
<option value="Cuba">Cuba</option>
<option value="Curaco">Curacao</option>
<option value="Cyprus">Cyprus</option>
<option value="Czech Republic">Czech Republic</option>
<option value="Denmark">Denmark</option>
<option value="Djibouti">Djibouti</option>
<option value="Dominica">Dominica</option>
<option value="Dominican Republic">Dominican Republic</option>
<option value="East Timor">East Timor</option>
<option value="Ecuador">Ecuador</option>
<option value="Egypt">Egypt</option>
<option value="El Salvador">El Salvador</option>
<option value="Equatorial Guinea">Equatorial Guinea</option>
<option value="Eritrea">Eritrea</option>
<option value="Estonia">Estonia</option>
<option value="Ethiopia">Ethiopia</option>
<option value="Falkland Islands">Falkland Islands</option>
<option value="Faroe Islands">Faroe Islands</option>
<option value="Fiji">Fiji</option>
<option value="Finland">Finland</option>
<option value="France">France</option>
<option value="French Guiana">French Guiana</option>
<option value="French Polynesia">French Polynesia</option>
<option value="French Southern Ter">French Southern Ter</option>
<option value="Gabon">Gabon</option>
<option value="Gambia">Gambia</option>
<option value="Georgia">Georgia</option>
<option value="Germany">Germany</option>
<option value="Ghana">Ghana</option>
<option value="Gibraltar">Gibraltar</option>
<option value="Great Britain">Great Britain</option>
<option value="Greece">Greece</option>
<option value="Greenland">Greenland</option>
<option value="Grenada">Grenada</option>
<option value="Guadeloupe">Guadeloupe</option>
<option value="Guam">Guam</option>
<option value="Guatemala">Guatemala</option>
<option value="Guinea">Guinea</option>
<option value="Guyana">Guyana</option>
<option value="Haiti">Haiti</option>
<option value="Hawaii">Hawaii</option>
<option value="Honduras">Honduras</option>
<option value="Hong Kong">Hong Kong</option>
<option value="Hungary">Hungary</option>
<option value="Iceland">Iceland</option>
<option value="India">India</option>
<option value="Indonesia">Indonesia</option>
<option value="Iran">Iran</option>
<option value="Iraq">Iraq</option>
<option value="Ireland">Ireland</option>
<option value="Isle of Man">Isle of Man</option>
<option value="Israel">Israel</option>
<option value="Italy">Italy</option>
<option value="Jamaica">Jamaica</option>
<option value="Japan">Japan</option>
<option value="Jordan">Jordan</option>
<option value="Kazakhstan">Kazakhstan</option>
<option value="Kenya">Kenya</option>
<option value="Kiribati">Kiribati</option>
<option value="Korea North">Korea North</option>
<option value="Korea Sout">Korea South</option>
<option value="Kuwait">Kuwait</option>
<option value="Kyrgyzstan">Kyrgyzstan</option>
<option value="Laos">Laos</option>
<option value="Latvia">Latvia</option>
<option value="Lebanon">Lebanon</option>
<option value="Lesotho">Lesotho</option>
<option value="Liberia">Liberia</option>
<option value="Libya">Libya</option>
<option value="Liechtenstein">Liechtenstein</option>
<option value="Lithuania">Lithuania</option>
<option value="Luxembourg">Luxembourg</option>
<option value="Macau">Macau</option>
<option value="Macedonia">Macedonia</option>
<option value="Madagascar">Madagascar</option>
<option value="Malaysia">Malaysia</option>
<option value="Malawi">Malawi</option>
<option value="Maldives">Maldives</option>
<option value="Mali">Mali</option>
<option value="Malta">Malta</option>
<option value="Marshall Islands">Marshall Islands</option>
<option value="Martinique">Martinique</option>
<option value="Mauritania">Mauritania</option>
<option value="Nepal">Nepal</option>
<option value="Netherland Antilles">Netherland Antilles</option>
<option value="Netherlands">Netherlands</option>
<option value="Norway">Norway</option>
<option value="Oman">Oman</option>
<option value="Pakistan">Pakistan</option>
<option value="Palau Island">Palau Island</option>
<option value="Palestine">Palestine</option>
<option value="Russia">Russia</option>
<option value="Rwanda">Rwanda</option>
<option value="St Barthelemy">St Barthelemy</option>
<option value="St Eustatius">St Eustatius</option>
<option value="St Helena">St Helena</option>
<option value="St Kitts-Nevis">St Kitts-Nevis</option>
<option value="St Lucia">St Lucia</option>
<option value="St Maarten">St Maarten</option>
<option value="Sweden">Sweden</option>
<option value="Switzerland">Switzerland</option>
<option value="Tonga">Tonga</option>
<option value="United Arab Erimates">United Arab Emirates</option>
<option value="United Kingdom">United Kingdom</option>
<option value="United States of America">United States of America</option>
<option value="Venezuela">Venezuela</option>
<option value="Wallis & Futana Is">Wallis & Futana Is</option>
<option value="Yemen">Yemen</option>
<option value="Zaire">Zaire</option>
<option value="Zambia">Zambia</option>
<option value="Zimbabwe">Zimbabwe</option>
</select>
<br>
<span id="st" style="visibility:hidden;">state</span>
<select id="state" name="state" size="1" style="visibility:hidden;">
<option value="Alabama">Alabama</option>
<option value="Alaska">Alaska</option>
<option value="Arizona">Arizona</option>
<option value="Arkansas">Arkansas</option>
<option value="California">California</option>
<option value="Colorado">Colorado</option>
<option value="Hawaii">Hawaii</option>
<option value="Idaho">Idaho</option>
<option value="Illinois">Illinois</option>
<option value="Indiana">Indiana</option>
<option value="Iowa">Iowa</option>
<option value="Kansas">Kansas</option>
<option value="Kentucky">Kentucky</option>
<option value="Louisiana">Louisiana</option>
<option value="Maine">Maine</option>
<option value="Maryland">Maryland</option>
<option value="Massachusetts">Massachusetts</option>
<option value="Michigan">Michigan</option>
<option value="Minnesota">Minnesota</option>
<option value="Mississippi">Mississippi</option>
<option value="Rhode Island">Rhode Island</option>
<option value="Washington">Washington</option>
</select>
</form>
</body>
</html>
This is it. Country list and state list are not complete
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