GDANSK, Poland (AFP/Reuters) - German leader Angela Merkel recalled World War II as the "most tragic chapter" in European history Tuesday as the continent marked 70 years since the conflict began on the shores of the Baltic. In a ceremony also attended by veterans of the 1939-45 war, Merkel joined leaders from countries including Poland and Russia at the former naval base which was the site of the first battle of the war on September 1, 1939. But while the ceremony underlined how Europe has been able to heal its divisions in the intervening years, old wounds were clear to see with fresh disputes over its historical legacy. "Seventy years ago today the German invasion of Poland opened up the most tragic chapter in European history," said Merkel. "The war unleashed by Germany resulted in immeasurable suffering to many peoples -- years of deprivation of rights, of humiliation and destruction." The Polish army, outnumbered by more than two to one, surrendered a little over a month after the battle on Gdansk's Westerplatte peninsula and a brutal Nazi occupation began. Almost six million Polish citizens perished in the war, half of them Jewish. Before heading to Poland, Merkel had said on German television that her country bore full responsibility for the conflict. "Germany attacked Poland, Germany started World War II. We caused unending suffering in the world. Sixty million dead ... was the result," the German chancellor said. Speaking at a pre-dawn ceremony before Merkel's arrival, Polish Prime Minister Donald Tusk also underlined how the memories and bitterness of 70 years ago still burned. "We are here to remember who in that war was the aggressor and who was the victim, for without an honest memory neither Europe, nor Poland, nor the world will ever live in security," said Tusk. In his address, Russian Prime Minister Vladimir Putin rejected criticism of Moscow's role just before World War Two during ceremonies on Tuesday marking the 70th anniversary of Nazi Germany's invasion of Poland. But Putin and Polish Prime Minister Donald Tusk agreed their countries' historians should work more closely to uncover darker parts of their shared past which still cloud relations 20 years after the collapse of communism in eastern Europe. "If we are going to speak objectively about history we must understand it does not have just one colour. It was diverse and a huge number of mistakes were made by all sides," Putin told a news conference after talks with Tusk in the resort of Sopot. "And all these actions created the conditions for the large scale aggression by Nazi Germany." Britain and France, bound to Poland by military pacts, declared war on Germany on September 3, 1939, each pulling their vast empires into the conflict. For Russia, what is known as the "Great Patriotic War" started on June 22, 1941, when the Nazis tore up their non-aggression pact and invaded. Suddenly the Soviet forces, from having been the occupiers of part of Poland, found themselves fighting against a common enemy. Around 8.6 million Soviet soldiers and 27-28 million civilians were killed in the war, which ended with Germany's crushing defeat in 1945. Merkel said that the continent's recovery since then should be a source of pride. "Europe ... has transformed itself from a continent of terror and violence into a continent of freedom and peace. That this was possible is nothing more and nothing less than a wonder," she said.