German Chancellor Angela Merkel said the lessons of two world wars have led her to take a firm stance against Russia's annexation of Ukraine's territory of Crimea.

At a May 28 opening of an exhibition on World War I, Merkel said Russia's annexation of Crimea was unacceptable, as it upset Europe's postwar order.

"Territorial integrity is the foundation pillar of our postwar European order," Merkel said and added, "If you start saying things like 'it's my right' and then just take something, you'll end up with an incredible calamity."

Merkel noted she would meet with Russian President Vladimir Putin next week at ceremonies marking the Western allies' invasion of Nazi-held France in June 1944.

"The lesson of the past is to learn cooperation instead of confrontation," she said.