During World War I, the Netherlands remained neutral. As a Dutch subject, Zelle was thus able to cross national borders freely. To avoid the battlefields, she travelled between France and the Netherlands via Spain and Britain, and her movements inevitably attracted attention. During the war, Zelle was involved in what described as a very intense romantic-sexual relationship with a Russian pilot, the twenty-five year old Captain Vadim Maslov serving with the French, whom she called the love of her life. Maslov was part of the 50,000 strong Russian Expeditionary Force sent to the Western Front in the spring of 1916. In the summer of 1916, Maslov was shot down and badly wounded during a dogfight with the Germans, losing his sight in both eyes, which led Zelle to ask for permission to visit her wounded lover at the hospital he was staying near the front. As a citizen of a neutral country, Zelle would not normally be allowed near the front. Zelle was met by agents from the Deuxième Bureau who told her that she would only be allowed to see Maslov if she agreed to spy on Germany. Before the war, Zelle had performed as Mata Hari several times before the Crown Prince Wilhelm, eldest son of Kaiser Wilhelm II and nominally a senior German general on the Western Front. The Deuxième Bureau believed she might be able to seduce the Crown Prince and learn valuable German secrets. In fact, the Crown Prince Wilhelm was an irresponsible playboy noted only for his love of women, partying and alcohol and his actual role as a commander was strictly minimal, but the French were not aware of this. The German government understandably was not anxious to publicise the fact that the man expected to be the next Kaiser was a spoiled and immature playboy who when not partying spent his time intriguing with far right-wing politicians to have his father declared insane, so he could become Emperor or at very least Regent, and instead promoted the image of the Crown Prince as a great warrior, the worthy successor to the august Hohenzollern monarchs who had made Prussia strong and powerful. Unaware that the Crown Prince did not have much to do with the running of Army Group Crown Prince or the 5th Army, the Deuxième Bureau offered Zelle one million francs if she could seduce him and provide France with good intelligence about German plans. The fact that the Crown Prince had, before 1914, never commanded a unit larger than a regiment, and was now supposedly commanding both an army and an army group at the same time should have been a clue that his role in German decision-making was mostly nominal. Zelle's contact with the Deuxième Bureau was Captain Georges Ladoux, who was later to emerge as one of her principal accusers.
In November 1916, she was travelling by steamer from Spain when her ship called at the British port of Falmouth. There she was arrested and brought to London where she was interrogated at length by Sir Basil Thomson, Assistant Commissioner at New Scotland Yard in charge of counter-espionage. He gave an account of this in his 1922 book Queer People, saying that she eventually admitted to working for the Deuxième Bureau. Initially detained in Cannon Street police station, she was then released and stayed at the Savoy Hotel. A full transcript of the interview is in Britain's National Archives and was broadcast, with Mata Hari played by Eleanor Bron, on the independent station London Broadcasting in 1980. It is unclear if she lied on this occasion, believing the story made her sound more intriguing, or if French authorities were using her in such a way but would not acknowledge her due to the embarrassment and international backlash it could cause.
In late 1916, Zelle travelled to Madrid, where she met with the German military attaché, Major Arnold Kalle, and asked if he could arrange a meeting with the Crown Prince. During this period, Zelle apparently offered to share French secrets with Germany in exchange for money, though whether this was because of greed or an attempt to set up a meeting with Crown Prince Wilhelm remains unclear. In January 1917, Major Kalle transmitted radio messages to Berlin describing the helpful activities of a German spy code-named H-21, whose biography so closely matched Zelle's that it was patently obvious that Agent H-21 could only be Mata Hari. The Deuxième Bureau intercepted the messages and, from the information it contained, identified H-21 as Mata Hari. The messages were in a code that German intelligence knew had already been broken by the French, leaving to the conclusion that the messages were contrived to have Zelle arrested by the French. General Walter Nicolai, the chief IC (intelligence officer) of the German Army had grown very annoyed that Mata Hari had provided him with no intelligence worthy of the name, instead selling the Germans mere Paris gossip about the sex lives of French politicians and generals, and decided to terminate her employment by exposing her as a German spy to the French.
Trial and execution
At her arrest
In December 1916, the French Second Bureau of the French War Ministry let Mata Hari obtain the names of six Belgian agents. Five were suspected of submitting fake material and working for the Germans, while the sixth was suspected of being a double agent for Germany and France. Two weeks after Mata Hari had left Paris for a trip to Madrid, the double agent was executed by the Germans, while the five others continued their operations. This development served as proof to the Second Bureau that the names of the six spies had been communicated by Mata Hari to the Germans.
On 13 February 1917, Mata Hari was arrested in her room at the Hotel Elysée Palace on the Champs Elysées in Paris. She was put on trial on 24 July, accused of spying for Germany, and consequently causing the deaths of at least 50,000 soldiers. Although the French and British intelligence suspected her of spying for Germany, neither could produce definite evidence against her. Supposedly secret ink was found in her room, which was incriminating evidence in that period. She contended that it was part of her makeup. Zelle's principal interrogator was Captain Pierre Bouchardon, the man who was to prosecute her at her trial, who grilled her relentlessly. Bouchardon was able to establish that much of the Mata Hari persona was invented, and far from being a Javanese princess, Zelle was actually Dutch, which he was to use as evidence of her dubious and dishonest character at her trial. Zelle admitted to Bouchardon that she had accepted 20,000 francs from a German diplomat in the Netherlands to spy on France, but insisted she only passed on to the Germans trivial information as her loyalty was entirely to her adopted nation, France. In the meantime, Ladoux had been preparing a case against his former agent by casting all of her activities in the worse possible light, going so far as to engage in evidence tampering.
In 1917, France had been badly shaken by the Great Mutinies of the French Army in the spring of 1917 following the failure of the Nivelle Offensive together with a huge strike wave, and at the time, many believed that France might simply collapse as a result of war exhaustion. In July 1917, a new government under Georges Clemenceau aka "le tigre" had come into power, utterly committed to winning the war. In this context, having one German spy for whom everything that went wrong with the war so far could be blamed was most convenient for the French government, making Mata Hari the perfect scapegoat, which explains why the case against her received maximum publicity in the French press, and led to her importance in the war being greatly exaggerated. The Canadian historian Wesley Wark stated in a 2014 interview that Mata Hari was never an important spy and just made a scapegoat for French military failures which she had nothing to do with, stating: "They needed a scapegoat and she was a notable target for scapegoating". Likewise, the British historian Julie Wheelwright stated: "She really did not pass on anything that you couldn’t find in the local newspapers in Spain". Wheelwright went on to describe Zelle as "...an independent woman, a divorcee, a citizen of a neutral country, a courtesan and a dancer, which made her a perfect scapegoat for the French, who were then losing the war. She was kind of held up as an example of what might happen if your morals were too loose”.
Zelle wrote several letters to the Dutch Ambassador in Paris, claiming her innocence. "My international connections are due of my work as a dancer, nothing else .... Because I really did not spy, it is terrible that I cannot defend myself". The most terrible and heart-breaking moment for Mata Hari during the trial occurred when her lover Maslov – by now a deeply embittered man as a result of losing his eyes in combat – declined to testify for her, telling her he couldn't care less if she was convicted or not. It was reported that Zelle fainted when she learned that Maslov had abandoned her. Her defence attorney, veteran international lawyer Édouard Clunet, faced impossible odds; he was denied permission either to cross-examine the prosecution's witnesses or to examine his own witnesses directly. Bouchardon used the very fact that Zelle was a woman as evidence of her guilt, saying: "Without scruples, accustomed to make use of men, she is the type of woman who is born to be a spy." Under the circumstances, her conviction was a foregone conclusion.
Mata Hari herself admitted under interrogation to taking money to work as a German spy. It is contended by some historians that Mata Hari may have merely accepted money from the Germans without actually carrying out any spy duties. At her trial, Zelle vehemently insisted that her sympathies were with the Allies and declared her passionate