<a href="https://en.wikipedia.org/wiki/World_War_II" id="mwlw" rel="mw:WikiLink" title="World War II">World War II</a>: The discovery of <a href="https://en.wikipedia.org/wiki/Mass_grave" id="mwmA" rel="mw:WikiLink" title="Mass grave">mass graves</a> of Polish <a href="https://en.wikipedia.org/wiki/Prisoner_of_war" id="mwmQ" rel="mw:WikiLink" title="Prisoner of war">prisoners of war</a> <a class="mw-redirect" href="https://en.wikipedia.org/wiki/Mass_execution" id="mwmg" rel="mw:WikiLink" title="Mass execution">killed</a> by Soviet forces in the <a href="https://en.wikipedia.org/wiki/Katyn_massacre" id="mwmw" rel="mw:WikiLink" title="Katyn massacre">Katyń Forest Massacre</a> is announced, causing a diplomatic rift between the <a href="https://en.wikipedia.org/wiki/Polish_government-in-exile" id="mwnA" rel="mw:WikiLink" title="Polish government-in-exile">Polish government-in-exile</a> in London and the <a href="https://en.wikipedia.org/wiki/Soviet_Union" id="mwnQ" rel="mw:WikiLink" title="Soviet Union">Soviet Union</a>, which denies responsibility.