<a href="https://en.wikipedia.org/wiki/World_War_I" id="mwYg" rel="mw:WikiLink" title="World War I">World War I</a>: The use of <a class="mw-redirect" href="https://en.wikipedia.org/wiki/Poison_gas_in_World_War_I" id="mwYw" rel="mw:WikiLink" title="Poison gas in World War I">poison gas in World War I</a> escalates when <a href="https://en.wikipedia.org/wiki/Chlorine" id="mwZA" rel="mw:WikiLink" title="Chlorine">chlorine gas</a> is released as a <a href="https://en.wikipedia.org/wiki/Chemical_warfare" id="mwZQ" rel="mw:WikiLink" title="Chemical warfare">chemical weapon</a> in the <a href="https://en.wikipedia.org/wiki/Second_Battle_of_Ypres" id="mwZg" rel="mw:WikiLink" title="Second Battle of Ypres">Second Battle of Ypres</a>.