Leaders of more than 40 countries gathered in Warsaw on Sunday for ceremonies marking the 80th anniversary of the invasion of Poland and the outbreak of World War Two.
German President Frank-Walter Steinmeier asked Poland for its forgiveness for Nazi "tyranny".
"I bow my head before the Polish victims of Germany's tyranny. And I ask forgiveness," Steinmeier said, speaking in German and Polish.
Around 250 guests from around the world including presidents, prime ministers, parliamentary speakers, foreign ministers and special envoys are expected to attend the event, in the city’s Pilsudski Square, the site of the Tomb of the Unknown Soldier.
US Vice President Mike Pence will replace Donald Trump, who has remained at home to oversee the response to the arrival of Hurricane Dorian.
Changing borders Polish President Andrzej Duda said the anniversary was very relevant to modern-day Europe as he met his Ukrainian counterpart, Volodymyr Zelenskiy, on Saturday.
"We must stress how very important it is that no one, in Europe or in the world, is allowed to change borders by force,” Duda said.
The pair agreed that sanctions ought to continue against Russia until Ukraine regains the territory it lost in Moscow's 2014 annexation of Crimea.
Zelenskiy said: "Today, I have sincerely thanked Poland and the whole Polish nation for all the support given during this difficult time of Russian aggression.”
Europe’s conflict September 1, 1939 marks the outbreak of World War Two, and the start of five years of Nazi occupation of Poland that led to the deaths of six million Polish citizens.
Early on that day, a German battleship opened fire on a Polish fort in Westerplatte, near Gdansk, while air forces bombed the central city of Wielun, killing hundreds.
At dawn on Sunday, Polish President Andrzej Duda and his German counterpart Frank-Walter Steinmeier will attend a remembrance service in Wielun.
Polish Prime Minister Mateusz Morawiecki and European Commission Vice-President Frans Timmermans will take part in a ceremony in Westerplatte.
Germany had agreed with the Soviet Union in secret to carve up Eastern Europe in the Molotov-Ribbentrop Pact.
Its invasion from the west was followed on September 17, 1939 by invasion by Stalin’s Soviet Union.
In the meantime, Britain and France had declared war on Germany.