GDANSK, Poland (CNN) -- At least 20 world leaders gathered Tuesday in Poland to commemorate the start of World War II 70 years ago -- a conflict in which 6 million Poles died. Russian Prime Minister Vladimir Putin (right) chats with his Polish counterpart Donald Tusk in Poland on Tuesday. Germany's pre-dawn invasion began when the battleship Schleswig-Holstein fired on the Westerplatte military base in Gdansk harbor on September 1, 1939. The attack set off a chain of events that eventually embroiled all of the world's major powers in the war. Polish World War II veteran Romuald Bardzynski, said: "I was a soldier on the front, a corporal in the cavalry. On September 16 I was injured. We were fighting against the Germans but I was shot by the Soviets. I was wounded three times.
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About 20 world leaders gathered in Poland to mark the start of World War II
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Attack set off chain of events that embroiled all of the world's major powers
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German Chancellor Angela Merkel, Russian PM Vladimir Putin at event
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Merkel says her country unleashed "endless suffering" by starting war
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"The Russians took half of Poland, and after the Polish fighting on all the world war fronts it turns out that after the war @placeholder was 77 sq km smaller!