Upon the outbreak of World War I Holmes was appointed as consultant neurologist to the British Expeditionary Forces. Working with his neurosurgical colleague Percy Sargent (1873–1933) in a field hospital they had set up, he had a unique opportunity for the investigation of the effects of lesions in specific regions of the brain on balance, vision and bladder function. While in France, Holmes met his future wife, Dr Rosalie Jobson, an Oxford graduate and an international sportswoman, to whom he subsequently proposed marriage while rowing on the Thames.