JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
          <div id="themes">
          <h2>Research Themes</h2>
            <ul>
              <li><a href="">Learn about our approach to the <strong>environment</strong></a><span><a href="#">Expand</a></span>
                <ul class="tier_2 hide">
                  <li><a href="">Project name numero uno goes here</a></li> 
                  <li><a href="">Project name numero dos goes here</a></li>
                  <li><a href="">Project name numero tres goes here</a></li>
                </ul>
              </li>
              <li><a href="">Learn about our approach to <strong>human health</strong></a><span><a href="#">Expand</a></span>
                <ul class="tier_2 hide">
                  <li><a href="">Project name numero uno goes here</a></li> 
                  <li><a href="">Project name numero dos goes here</a></li>
                  <li><a href="">Project name numero tres goes here</a></li>
                </ul>
              </li>
              <li class="last"><a href="">Learn about our approach to <strong>national defense</strong></a><span><a href="#">Expand</a></span>
                <ul class="tier_2 hide">
                  <li><a href="">Project name numero uno goes here</a></li> 
                  <li><a href="">Project name numero dos goes here</a></li>
                  <li><a href="">Project name numero tres goes here</a></li>
                </ul>
              </li>
            </ul>
          </div><!-- // end themes -->

CSS

ul li ul { display: none; }
ul li span a { padding: 6px; }

JavaScript

$('ul li span a').live('click', function () {
  $(this).closest('li').siblings('li').find(".tier_2").slideUp();
  $(this).closest('li').children('ul').slideToggle();
});