Sixty eight years ago this week, as the Second World War erupted, Poland suffered a double tragedy: invasions by German forces from the west, and by the Red Army from the east, and a war crime which would poison international relations for decades.
A secret pact between Berlin and Moscow saw Poland divided up, with each side occupying half the country, strangling the Polish resistance in a vice.
Four years later though, Hitler reneged on the pact and marched on Moscow. His advancing troops discovered the remains of thousands of Polish soldiers in a mass grave inside the Soviet Union. It gave the Nazis the chance to drive a wedge between Moscow and its Allies in London and Washington.
They filmed the horrors of Katyn in grisly detail and made it public. Moscow denied it and said the Nazis were trying to cover up an atrocity committeed by their own troops.
Despite evidence of Moscow’s guilt, Britain and the United States chose to accept the Soviet line, so as not to weaken the war effort.
It took until the fall of the USSR for Moscow to admit its guilt. In 1992, the then-Polish president Lech Walesa accepted official documents detailing the crime.
It was on Stalin’s personal orders that the Soviet secret police carried out their murderous work.
A year later, then-Russian President Boris Yeltsin visited Katyn to pay his respects. But his absence two years later at a similar ceremony, despite a personal invitation from Mr Walesa, infuriated Warsaw, and did nothing to improve the atmosphere between the two countries.
Russia still refuses to hand over other top secret documents on the massacre.
Warsaw has just one goal now: to de-classify the papers, and get Moscow to admit its guilt of not just a war crime, but attempted genocide.