Poland was essentially split in half during WWII between Russia and Nazi Germany with hundreds of thousands Polish soldiers made prisoners of the Soviet Army. Most of these soldiers were released, but approximately 22,000 officers were rounded up and killed by the Russian security services. Most of the killings occurred near the Katyn Forest, which gives the event its name. When the German army invaded through Poland they discovered the bodies and turned it into a press coup for themselves and invited an independent panel to study the war crimes. It's a gruesome sort of irony and, though the Germans eventually lost, it forever distanced the Soviets from the hearts of most Poles. All this goes to explain Alfisted's COTD-winning quote in this post.
Sweet revenge for all of the poles impaled by Russians over the centuries.
Photo Credit: Wally G @ Flickr