In Rome during World War II a plain white line was painted along the streets that ran by the Vatican. In 1944, during the early days of the German occupation, it marked the point where the Holy See's authority ended and Nazi rule began. It had been painted on the instructions of Obersturmbannführer (Lieutenant-Colonel) Herbert Kappler, the head of the Gestapo in Rome, who ruled the city with fear.
Kappler's street painting was a physical attempt to remind Romans who was in charge but it may also have been directed at one man - his rival Msgr Hugh O'Flaherty. The two men were engaged in a deadly game of "hide and seek" as the charismatic Kerry priest was running an escape operation for Allied servicemen and Jewish civilians from the confines of his Vatican office. After Msgr O'Flaherty hid the escapees it was Kappler's job to find them.
During the occupation Msgr O'Flaherty managed to outwit Kappler by using fake documents and secret communication channels and by dodging raids by German soldiers. The monsignor succeeded in evading capture and his story and his intriguing relationship with Herbert Kappler are set out in my book, Hide and Seek (details page 18).
Hugh O'Flaherty was from an Irish nationalist background. His views were formed when, as a young student in Limerick, he saw atrocities being carried out by Black and Tan soldiers from Britain and a number of his friends were killed. When the war began in 1939 he was understandably careful to avoid taking sides. He told one colleague: "I don't think there is anything to choose between Britain and Germany."
The Irishman's views changed as the war developed, particularly after he learned of the violence being inflicted on Jews. He also began to visit Allied prisoners being held in harsh conditions in Italian jails and in 1943 he began to offer shelter to Allied servicemen who turned up at the Vatican looking for sanctuary.
By 1944 Herbert Kappler had established a ruthless regime in Rome and such was the German's desire to stop his Irish rival that he tried to kidnap and kill the monsignor and even placed a bounty of 30,000 lire on his head. An ambitious high-flyer, Herbert Kappler was highly thought of by Adolf Hitler.
Throughout the Nazi occupation, however, messages sent by Kappler from Rome to Germany were intercepted by the Allies and the decoded messages that have now been declassified are available in the National Archives in Washington. The documents reveal how Kappler would round up Jews, how he helped to rescue Benito Mussolini, and what he thought of the Catholic Church and the Vatican. Using the decodes, some of which have not been published before, we are able to build up the most comprehensive picture to date of Kappler's behaviour.
It is with the events of March 1944, however, that the Gestapo chief will forever be associated. After the Resistance killed 33 German soldiers in a bomb attack Hitler was enraged and demanded a revenge attack to "make the world tremble". Kappler drew up the plans to do so. Then Kappler and his men killed 335 people in the Ardeatine Caves, a labyrinth of tunnels outside the city. It was one of the worst atrocities committed on Italian soil during the Second World War.
After the war, Kappler was sentenced to life imprisonment with no parole for his role in the Ardeatine Caves massacre and was told he would never be freed. However, within months Italy's most famous prisoner wrote to his old rival. He invited Msgr O'Flaherty to visit him and, within days, the Kerry priest arrived to meet and talk with his former foe. Their meetings became regular affairs and, according to Msgr O'Flaherty's friends, they discussed religion and literature.
The classical singer Veronica Dunne, who knew the monsignor, remembers him meeting Kappler. She says the Kerry priest enjoyed the visits. "He took a great liking to him. He used to joke: 'Here I am, this man who had 30,000 lire over my head for information and now we are sort of pals'." It seems the feeling was mutual as Kappler would describe Msgr O'Flaherty as "a fatherly friend". At this stage Kappler, who had been raised as Protestant, was considering becoming a Catholic and was influenced by his former rival.
A nephew of the monsignor, the former Irish supreme court judge who is also called Hugh O'Flaherty, says his uncle urged Kappler to delay his conversion until the trial was concluded. "My uncle advised him that it would be construed as if he was trying to curry favour," he says.
Kappler waited until he was sentenced and then called on the monsignor to visit him. The two men prayed together and then Msgr O'Flaherty received Kappler into the Catholic Church. In a matter of minutes, Italy's most notorious Nazi was welcomed into the faith by the very man he had tried to kill.
So where had this new-found faith come from? Had Kappler turned to God simply because he was facing a life sentence and it was convenient to pretend he was remorseful and wanted to seek public sympathy? Or was there an influence in his life which made him genuinely think in a way that he had never done before?
What is clear is that O'Flaherty was a huge influence on the former Gestapo chief and had become a close confidant. It is worth stressing that the monsignor was not a proselytising cleric and would not have pushed Kappler towards Catholicism, even though they discussed his desire to convert.
According to prison letters discovered by the journalist Pierangelo Maurizio it appears that Kappler's conversion took place around 1949 but the story didn't become public until 1959. Typically the modest and self-effacing monsignor played down the event. He told one inquisitive reporter: "That is not news. That is something which occurred a long time ago."
Msgr O'Flaherty was awarded a CBE and a US Medal of Freedom for his wartime efforts and died in County Kerry in 1963.
Herbert Kappler remained in prison in Italy throughout the 1960s and much of the 1970s. He re-married in 1972 and his new wife, Anneliese, campaigned to have him released and returned to Germany, but the Italian authorities refused. It seemed that Kappler was destined to end his days on Italian soil. However, in 1975 his circumstances changed when he was diagnosed with stomach cancer and transferred to a military hospital in Rome.
Anneliese Kappler continued to campaign for his early release with no success so on an August evening in 1977 she took matters into her own hands. She dramatically smuggled her husband out of his hospital room and placed him in a waiting car. She then drove him out of the hospital complex and took him back to Germany where he died in 1978.
Part of this story was dramatised in the 1983 film The Scarlet and the Black. But the wartime duel is only one dimension of this extraordinary narrative: a rivalry that was forged in wartime which became a friendship created in peacetime. It remains one of the most fascinating stories to emerge from the Second World War.
Stephen Walker is a political reporter with BBC Northern Ireland. His book Hide and Seek: The Irish Priest in the Vatican Who Defied the Nazi Command is published by HarperCollins, 352pp, 978-0-00732-027-1 and is available from Freedom Publishing. This article first appeared in the London Catholic Herald.