In a deeply personal and historic address, he told members of the Bundestag in Berlin on Thursday that the “security of Europe has been threatened” and it was a “sacred responsibility” to unite in support of Ukraine after Vladimir’s Putin’s invasion.
He said: “The scourge of war is back in Europe. The unprovoked invasion of Ukraine has inflicted the most unimaginable suffering on so many innocent people.
“Countless lives have been destroyed; freedom and human dignity have been trampled in the most brutal way. The security of Europe has been threatened, together with our democratic values.”
Charles, on the second day of his state visit, also spoke of how Britain and Germany had built a prosperous and sustainable future since the Second World War through shared values and with deep cultural and personal ties of kinship.
He continued: “Heeding the lessons of the past is our sacred responsibility, but it can only be fully discharged through a commitment to our shared future.
“Together we must be vigilant against threats to our values and freedoms, and resolute in our determination to confront them. Together we must strive for the security, prosperity and well-being that our people deserve.”
Speaking in both German and English, Charles said he could "hardly begin to express the pride" he felt in the relationship between the two countries and spoke fondly of his "cherished" family ties to Germany.
After a quip about the Lionesses' victory over Germany at Euro 2022, the monarch hailed the teams’ promotion of gender equality and how they “inspired the world” as "just one example of how our countries, together, can offer a compelling example to the world".
He also thanked the country for its kindness following the death of his mother, the late Queen.
He said: "Today, it gives me particular pride to be with you ... and to renew the special bond between our two countries.
"This friendship meant so much to my beloved mother, the late Queen, who often spoke of the 15 official visits she made to Germany, including her five state visits.
"The first of those, in 1965, came when our continent was still deeply scarred by war, and the trauma of conflict. Hers was the wartime generation, and like my father, the Queen had served in uniform.
"That my parents' 11-day tour of Germany should prove to be a pivotal moment in the reconciliation between our nations was, therefore, a matter of great personal significance to them both.
"My mother understood the immense achievement that reconciliation represented, and in returning to Germany time and again, she was determined to play her own part. It is, perhaps, for this reason that Her late Majesty won a particular place in the affection of the German people."
He touched on climate change leadership before concluding: "In the long and remarkable story of our two countries, there are many chapters yet unwritten. Let us fill these with the restless pursuit of a better tomorrow. The legacy of our past, and the great promise of our future, demand nothing less."
The Bundestag, the parliament of the federal republic, suspended sitting for the King’s speech, which was broadcast live across the nation. It was Charles’s second address in the Reichstag building.
In November 2020, as Prince of Wales, he gave a speech at a German War Graves Commission event honouring the victims of war.
Later on Thursday the King, who is accompanied by the Queen Consort, visited a joint German-British military unit to see their bridge-building amphibious vehicles.
Germany has welcomed more than a million Ukrainian refugees and Charles also toured the Tegel Refugee Centre to meet some of them. Camilla was visiting the Refugio House community centre, a meeting place for locals and new Berlin residents, including refugees.
Charles was given a tour of the Ukraine Arrival Centre by German President Frank-Walter Steinmeier and stopping to speak to families registering after just arriving, asked: "Was it very difficult to get out of Ukraine? Are you pleased to be here?"
Charles, who is midway through a state visit to Germany with the Queen Consort, took up the offer of a game of table football in the refugee centre's games room and joked: "I remember trying this when I was younger."
But when he conceded a goal, he replied: "Amazing, two hands you are the experts."
Speaking afterwards through a translator Olena Ochkiviska, 40, said: "I told him everyone was caring after us and they are working on the negatives.
"He said that he was praying for us all. I'm the luckiest lady in the world."
Kleopatra Tummler, operations manager of the refugee centre, who also worked 15 years as Take That's tour manager, said afterwards: "He really showed interest in everyone's stories.
"He learned that some said there were not enough cellars in Ukraine for people to hide.
"There was a family who had just arrived and registered here who he stopped to talk to."
Eslewhere, the Queen Consort and First Lady Elke Budenbender met opera singers from the Komische Opera Berlin.
The King and Queen Consort’s state visit, the first of Charles’s reign, began on Wednesday with a ceremonial greeting at Berlin’s Brandenburg Gate.
They then attended a state banquet, where Charles, 74, reiterated Britain’s desire to stand with Germany in advancing democratic values.
He said: “This is epitomised so clearly today as we stand together with Ukraine in defence of freedom and sovereignty in the face of unprovoked aggression.”
Camilla, 75, wore a black evening dress with silver embroidery by Bruce Oldfield and the Boucheron diamond tiara, a favourite of the late Queen Mother that was also worn by Queen Elizabeth.
The Queen last visited Germany in 2015 when she travelled to Berlin and Frankfurt and also saw the former Nazi concentration camp at Bergen-Belsen.
In his speech at the banquet, Charles spoke of the friendship between the two nations, adding: “It was a friendship which mattered greatly to my mother, the late Queen, who cared deeply about the bond between our two countries.”
On Friday Charles and Camilla will visit Hamburg, where he will become the first British monarch to lay a wreath to the German victims of allied air raids.
Their tour had planned to take in France but the visit was cancelled due to mass protests over raising the pension age.