German Chancellor Angela Merkel said yesterday she had taken a firm stance against Russia's annexation of Crimea in part due to the lessons of two world wars, but added it was vital to keep talking with Moscow.

Speaking to students at the opening of an exhibition on World War One, Merkel said her insistence that Russian President Vladimir Putin join her and other Western leaders in Normandy, France on June 6 for ceremonies marking the 70th anniversary of the D-Day landings was to avoid past mistakes.

"There are times where you have no desire to talk any more, such as with Russia now," said Merkel, who speaks Russian and regularly talks to Putin on the phone.

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"I force myself to talk. I'm surprised every time to see the other side's point of view."

Merkel said Russia's annexation of the Crimea was an unacceptable move because it upset Europe's postwar order - even though a 54 per cent majority of Germans expressed understanding, in a March opinion poll, for the annexation.

"That's why I'm so strict when it comes to the Crimea issue," she said.

"Territorial integrity is the foundation pillar of our postwar European order. If you start saying things like 'it's my right' and then just take something, you'll end up with an incredible calamity. That doesn't work."

Putin will meet Merkel, Barack Obama and other leaders - for the first time since the outbreak of the Ukraine crisis - in France to mark the anniversary of the D-Day landings against Nazi Germany in 1944 that helped turn the tide of World War Two.

"I worked very hard to ensure that the Russian president would come," Merkel said. She also said: "The lesson of the past is to learn co-operation instead of confrontation."

In unusually candid comments, Merkel said she was astonished by people's attitudes at the outbreak of World War One.

"It always amazes me to see that there could be such total enthusiasm that came in 1914," she said.