Between 1906 and 1911, Hammersley commanded the 3rd Brigade, Aldershot Command, but was relieved of his position due to repeatedly showing signs of shell shock. Instead he was given command of Northumbrian Division in September 1911. Despite his shell shock, upon the outbreak of the First World War, he was put in command of West Lancashire Division in August 1914 and shortly thereafter of the newly formed 11th (Northern) Division, part of Kitchener's volunteer army. In this capacity, Hammersley commanded the Landing at Suvla Bay by his division during the Gallipoli Campaign. However, his ability to oversee such an operation was subsequently called into question, and the Dardanelles Commission openly criticised his command. The orders given by General Hammersley were deemed to be confused and the work of his staff defective. On 23 August 1915, he was removed from the front-line in a state of collapse and was replaced by Major-General Edward Fanshawe. He was invalided back to England, suffering from battle fatigue. Lord Kitchener had warned that he should be watched to ensure that "the strain of trench warfare is not too much for him".