In September 1916, during the First World War, Christie enlisted in the British Army; he was called up on 12 April 1917 to join the 52nd Nottinghamshire and Derbyshire Regiment to serve as an infantryman. In April 1918, the regiment was despatched to France, where Christie was seconded to the Duke of Wellington's (West Riding) Regiment as a signalman. That June, he was injured in a mustard gas attack and spent a month in a military hospital in Calais. Christie claimed this attack left him permanently unable to speak loudly. Later in life, he also claimed the attack had rendered him blind and mute for three-and-a-half years. His period of muteness was, he claimed, the reason for his inability to talk much louder than a whisper for the rest of his life. Ludovic Kennedy points out that no record of Christie's blindness has been traced and that, while he may have lost his voice when he was admitted to hospital, he would not have been discharged as fit for duty had he remained a mute. His inability to talk loudly, Kennedy argues, was a psychological reaction to the gassing rather than a lasting toxic effect of the gas. That reaction, and Christie's exaggeration of the effects of the attack, stemmed from an underlying personality disorder that caused him to exaggerate or feign illness as a ploy to get attention and sympathy.