The actor, famous for his portrayal of Tom Barnaby in Midsomer Murders, spent 12 years in the Channel Islands in the 1980s and early 1990s while filming the television series Bergerac.
Aware of the islands' occupation by the German forces from 1940 to 1945 due to the reminders along the cliffs and shoreline where remains of bunkers, observation towers and other wartime fortifications still exist, Nettles (71) was always interested in exploring it further.
The documentary The Channel Islands at War, which used archive film footage and interviews with historians and survivors, turned out to be much more complex and complicated than he ever imagined.
''It was a particularly, awful, nasty time.''
Hundreds of inmates died in Alderney slave camps and many island residents died in air raids.
His aim was to delve into the accusations made of the community at the time, of collaborating with the Germans, being too cowardly to resist and for collaborating and conniving in sending Jews to the Holocaust.
''Despite all the years that have passed, they still get very agitated - it still sits below the surface.''
This was highlighted by a series of letters he received threatening bodily harm if he stepped on the island of Guernsey again.
The islanders had even been called ''lily-livered collaborators'' by Winston Churchill at the time and that accusation had stuck.
He spoke to some of the survivors of the occupation, English and German, and those who did resist.
''The problem is, in an occupation the room to manoeuvre is limited primarily because those in power have a gun in their hand.''
Island authorities were in a terrible position having to do everything the Germans demanded of them as they were threatened with the SS coming in to take over.
''That wasn't a pleasant prospect.''
The grounds for the claims of conniving were based on the lack of protest made when three Guernsey Jewish women were shipped to Auschwitz in 1942.
''Those that knocked on the door were Guernsey police officers, those that made the lists were Guernsey bureaucrats.
''It was only three people but that was three too many.''
Despite the the threats, there was a brave network of people who did resist but not in an obvious way, he said.
''On a small island, there was nowhere to hide. There were more Germans per square mile on the island than there was in Germany then.
''Resistance was very difficult when raising an eyebrow could get you shot.''
Despite raising awareness of a difficult time in the islands' history, Nettles loves the area and regularly returns to visit his daughter and grandchildren who now live there.
''I'm an honorary Jersey man. There is nowhere else I'd rather be.''
His next project is a documentary on art, poetry, literature and music produced from the hardship of World War 1.
• The Channel Islands At War premieres tonight at 7.30pm on the History Channel.
