In 1912, he was appointed GSO1 of the Meerut Division. At the outbreak of World War I in 1914, an Indian corps was sent to France; Jacob went with the Meerut division. He became the only Indian officer to remain in France and rise to a high command there. Just before Christmas of 1914, a critical situation arose on the front of the Indian corps, which was badly shaken by a German attack. When his seniors failed, Jacob stepped into the breach and by his personal example as well as by his tactical grasp, went far to retrieve the situation.
At the beginning of 1915, he was appointed to command the Dehra Dun Brigade, and led it at Neuve Chapelle in March. His practical sense and moral courage were shown in a way that, after a personal reconnaissance, he countermanded an assault that he saw would be in vain—in contrast with other commanders. In May, he was given the Meerut division and committed to the fruitless and costly subsidiary action of Pietre at the time of the battle of Loos. In November, when the Indian Corps was preparing to leave the Western front, he was appointed to take over the 21st Division of the "New Armies" on November 18, 1915. He brought his command to excellent fighting pitch by precept and example. He was created CB in this year.
In September 1916, he was appointed to command the 2nd corps in the Fifth Army. During the battle of the Somme, he carried Thiepval by an admirably planned assault. They performed well at the Acre operations and in the pursuit of the Germans to the Hindenburg Line in 1917.
Having been promoted to lieutenant-general in 1917, he commanded a corps of the Rhine army for a time. In 1920, he returned to India as Chief of the General Staff. In the course of that year, he was promoted to general and appointed ADC to King George V. In 1924, he returned home to England, but then in November was given the Northern Command in India. When Lord Rawlinson died in the following year, he acted temporarily as commander-in-chief. Many anticipated that the succession would fall upon him, and this was believed to be the desire of the military authorities in London, but they failed to make it sufficiently clear. Thus the choice fell on Sir William Birdwood instead, and in November Jacob returned home again to take up the appointment of Secretary to the Military Dept of the India Office. He was promoted field marshal in November 1926 and remained at the India Office until May 1930. His last official post was that of Constable of the Tower of London, which he held from 1937 to 1943. He was appointed KCMG in 1919, KCSI in 1924, GCB in 1926 and GCSI in 1930.
The couple had one son, Edward Ian Claud Jacob, who later became Assistant Military Secretary of the War Cabinet and Director-General of the BBC. Their daughter, Aileen Swinton Jacob, born on 5 August 1895, died on 14 January 1907.