"We know who started that murderous war," Gauck said on Friday at a ceremony in the eastern city of Dresden commemorating the 70th anniversary of the city's bombing.
On Feb 13, 1945, Dresden, once known as Florence on the Elbe, was bombed to ruins by the Allied air force. At least 25,000 people were estimated to be killed.
"We are no longer prepared to deny or defend transgressions and crimes committed in our country's name," Xinhua quoted the President as saying.
"A country, which committed an atrocity like genocide, could not expect to emerge unpunished and undamaged from a war, which it itself unleashed," Gauck said.
"We remember all those who lost their lives at the time as victims of the war and violence, not only in Dresden, but also elsewhere," said Gauck, addressing guests including representatives from Britain, Poland, Russia and other countries.