Marthe Mathilde Cnockaert, daughter of Felix Cnockaert and Marie-Louise Vanoplinus, was born on 28 Oct 1892 in Westrozebeke, West Flanders, Belgium. She died there on 8 Jan 1966.
She married John "Jock" McKenna, a British army officer from whom she separated around 1951.
Martha began studying at the medical school at Ghent University, but her studies were interrupted by the outbreak of WWI.
German troops razed the village in August 1914, burning her home down and temporarily separating her family. Having trained as a nurse, she gained a job at a German military hospital located in the village, where she was valued for her medical training and her multi-lingual skills, speaking English and German as well as French and Flemish. She was awarded the Iron Cross by the Germans for her medical service.
In 1915, she was transferred to the German Military Hospital in Roulers, where she was reunited with her family who had also moved there after the destruction of their home. Around this time, she was approached by a family friend and former neighbour, Lucelle Deldonck, who revealed to Cnockaert that she was a British intelligence agent, and wished to recruit her to an Anglo-Belgian intelligence network operating in the town.
For two years, Cnockaert used her cover as a nurse and her frequent proximity to German military personnel -- at both the hospital and as a waitress at her parents' cafe -- to gather important military intelligence for the British and their allies, which she passed on to other agents in local churches. She mostly worked with two other female Belgian spies: an elderly vegetable seller codenamed "Canteen Ma", and a letterbox agent codenamed "Number 63", both of whom helped her relay messages to and from British General Headquarters.
Her exploits during the war included destroying a telephone line which a local priest was using to spy for the Germans; and obtaining details of a planned but cancelled visit by Kaiser Wilhelm II for a British aerial attack. At one point, her German lodger, Otto, tried to recruit her to spy on the British. Cnockaert attempted to relay harmless but seemingly important information to him for a short time, but when operating as a double agent became too difficult, she arranged for him to be killed.
She discovered an unused sewer tunnel system located underneath a German ammunition depot, and placed the explosives herself to destroy the ammo dump. This operation led to her exposure and capture when she lost her watch, engraved with her initials, while placing the dynamite. In November 1916, she was sentenced to death for her espionage, but her sentence was commuted to life imprisonment due to her Iron Cross honour. She served two years in a prison in Ghent, and was released in 1918 when the Armistice with Germany was declared, ending the war.
Cnockaert was portrayed by Madeleine Carroll in I was a Spy, the 1933 film based on her memoirs.