The connection between the British Royal family and Germany existed long before the start of World War II. This diplomatic link was confirmed in the most scandalous way by discovering the incriminating Marburg files in 1945.
Still, even within the British monarch, no royal connection to Nazi Germany was stronger than Edward VIII's.
The link between Edward VIII and Nazi Germany was so solid and embarrassing that the country went to many lengths to keep the details from the public. Apparently, Adolf Hitler had been courting Edward for a while, and he even managed to send Germans to Britain to get intelligence on the royal family. The plan seemed to be working, based on the fact that Edward VIII ended up visiting Hitler in Germany.
Edward VIII, who was also reportedly anti-Semitic, even visited a concentration camp while there, although that was before the atrocious horrors there began.
After the trip, Hitler determined that the Duke of Windsor was an advocate of the Nazis who could be helpful in the future. However, the plan to have him as a puppet king began three years after the infamous visit. It was documented in the scandalous Marburg files.
King Edward VIII Abdicates The Throne

Edward VIII was King George V and Queen Mary's eldest child. On January 20, 1936, he became the king of the United Kingdom after his father passed away. King George V was grandfather to Queen Elizabeth II, so Edward VIII was the Queen's uncle.
In 1930, Prince Edward met Wallis Simpson, a divorced woman from the United States. Before long, he was in love, but he could not marry her as the Church of England did not allow royals to marry divorcees.
As king, he would have to head the Church of England. His only option was to forget about his lover and marry someone else or abdicate the throne. King Edward VIII decided he would not leave the love of his life, and so he became the first British royal to abdicate the throne.
After abdicating the throne, his brother, King George VI, took over. King George VI became the father of the current Queen, Elizabeth II, and was Prince Charles' grandfather.
Edward VIII became Duke of Windsor and married Simpson in France in June 1937. After Edward VIII married Simpson, she was not awarded the "royal highness" title the husband enjoyed. The decision was a massive blow to the Duke.
The couple lived in France and often traveled to Europe, and Germany was no exception. In October of the same year, the newlyweds visited Adolf Hitler as revered guests of the Nazis.
Unfortunately, this was just the beginning of the problems the Duke and Duchess brought upon themselves, fellow royals, and the British government.
The visit to Germany was nothing compared to the incriminating evidence of his links to the Nazi regime evidenced by the Marburg files, which were discovered soon after the war.
His abdication actually came as a relief to the British government, given his potentially treasonous connection to Nazi Germany. King Edward VIII was considered a Nazi sympathizer, which made him a huge liability to his family and country during and after the Second World War.
What Are The Marburg Files?

The Marburg files are top-secret records consisting of over 400 tons of archives. They came from the Foreign Minister in Nazi Germany, Joachim von Ribbentrop.
American troops found the files at Schloss Marburg in May 1945. The top-secret documents were located near Harz Mountains but compiled at Marburg Castle in Hesse.
American soldiers were going through the outskirts of Degenershaussen Estate when they found papers scattered on the side of the road close to numerous abandoned military vehicles. First Lieutenant David D. Silberberg found the documents, which had been signed by the Nazi foreign minister, Joachim von Ribbentrop. Silberberg did further investigations.
Apparently, there were orders to destroy the documents. The person given the responsibility, Von Loesch, had chosen to keep some of the documents and therefore buried about 400 tons of material.
Von Loesch had hidden the documents hoping to use them as leverage if he was charged as a war criminal.
After a close inspection, it was found that about 60 pages of detailed correspondence between the Duke of Windsor and Nazi Germany. These documents were then named the Windsor File.
Among the incriminating bits of information, the Americans found in the Marburg files was a detailed plan by Germans known as Operation Willi.
Operation Willi As Detailed In The Marburg Files

Operation Willi was a secret plan by the Nazis to convince the Duke of Windsor to join them in Germany. The Germans also planned on telling him of a fake plan by his brother, King George VI, and Prime Minister Winston Churchill to assassinate him.
They also planned on persuading him to stage a kidnapping so that they could blackmail the UK into surrendering. The idea, which did not succeed, was to make the Duke and Duchess of Windsor work beside Hitler and the Nazis as a way to forge peace between Germany and Britain and have him reinstated as king.
To the Germans, the Duke was clearly an ally, and they saw the fact that he was ostracized by his own as something they could use to their benefit.
Apparently, the Duchess fully supported the kidnapping idea, mainly because it meant the Duke would be king once more. The couple was particularly excited that they could circumvent the British constitution and ascend to the throne. The law could not allow them to assume the throne after the abdication, but the dynamics of war could bring about changes that the constitution could not.
Edward VII Duke Encouraged Nazis To Bomb The UK To Make It "Ready For Peace"

The Marburg files revealed that, in a telegram, the Duke believed that there would not have been a war if he had remained on the throne. The papers also alleged that he was a firm fan of a peaceful compromise with Germany.
Even more scandalous was a piece of evidence that revealed that the Duke believed that continuing to bomb the UK heavily would make "England ready for peace."
According to the Marburg files, the Nazis at one point tried to use the Duke and his family as leverage in a plan to try and get control of the British government. This was after France ended up under the control of the Nazis and the Duke and Duchess traveled to Madrid, Spain.

The government knew about the Duke's questionable links to Nazi Germany and made him Governor of the Bahamas to keep him out of Europe. He was in The Bahamas from 1940 to 1945. After the end of the war, he lived in France with short visits to England.
When the Marburg files were discovered, Winston Churchill and the Crown tried their best to ensure it did not go public. The prime minister was determined to ensure that all traces of Nazi telegrams were destroyed, as well as information about plans to reinstate Edward VIII as a king.
Churchill feared that the public would learn of the information and think the Duke was acting in a disloyal manner. That is why he urged the US president, Dwight D. Eisenhower, to keep the Windsor part of the Marburg files secret for at least 10 to 20 years, and the US president agreed.
The Marburg Files Today

The Windsor File proved that the Crown was sympathetic to the Nazis. That is why the royal family was so keen on keeping the whole thing a secret to avoid a bad perception of the British Crown.
The Marburg files are referenced in the hit Netflix show, The Crown.
According to historians, the prince mainly visited Hitler to show his new bride that she was important and part of important state affairs.
Still, it was apparent that the British government knew of Edward VIII's ambivalence towards Nazi Germany long before discovering the Marburg files. That is why they stopped briefing him on national security matters based on the belief that he was capable of treason. Based on the Marburg files, they had every reason to be concerned.
Eventually, the Marburg files were made public in 1957, and the Duke said they were "complete fabrications." People can't help but imagine that he would have been sympathetic to the Nazis if he would have remained at the helm of the British monarchy, changing the course of the war and possibly the history of the world as we know it today.
Edward lived a life of luxury in France, although he was still ostracized. He was never tried for treason, and he passed away in 1972 in France.