He was brought up at Radlett, Hertfordshire, but it was the effect of the First World War on his father's homeland that was crucial to his decision to become a conscientious objector. "The whole story of World War I was so overwhelming that I think many of us said we must never be part of this again," he later recalled

At the outbreak of war, Cammaerts registered as a conscientiousobjector and was sent to work on the land in Lincolnshire. But he was Persuaded by Rée to suspend his objection to military service, Cammaerts was recruited in July 1932 by Selwyn Jepson to join SOE and they realised he was the ideal man for some of the tasks SOE wanted.

Captain Cammaerts was assigned to F Section, which organised resistance operations in France, and was flown into Occupied France by Lysander to work with the Carte network, a putative group of resistants across southern France.  Shortly after his arrival, the acting leader of Carte sent a Colonel Heinrich Verbeck to see Cammaerts with a letter of recommendation assuring him that Heinrich Verbeck claimed to be a colonel in the German military intelligence service (the Abwehr) who was anri-Nazi and wanted to work with the resistance.

"Cammaerts most wisely distrusted the smell of the whole affair," the SOE official history records. Verbeck was, in fact, an Abwehr sergeant, Hugo Bleicher, who had infiltrated a large spread of French resistance networks.

There were already concerns over the work of another SOE officer, the French pilot Henri Dericourt, who it later emerged, was supplying Bleicher with details of operations and personnel.

Cammaerts immediately cut off all contact with Carte and its
successor organisation, Donkeyman, and moved to the mountains of south-east France to begin creating his own network. Using the code- name Roger, and the cover of a French teacher recuperating after jaundice, he rarely stayed in one place for more than one or two nights. He set up an astonishingly loyal network of resistants (codenamed Jockey) that would eventually stretch from the Mediterranean north to Lyon and across to the Swiss and Italian borders.

Cecily Lefort, one of his two main lieutenants, was captured and tortured by the Gestapo in September 1943, but the system Cammaerts had set in place ensured that the network survived.  Cecily Lefort was replaced, by the Polish countess Krystyna Skarbeck-Gizycka, otherwise known as Christine Granville, until July 1944, shortly before the Allied invasion of southern France.