Pilcher spent his early career as an infantry officer, first seeing active service on colonial campaigns in Nigeria in the late 1890s followed by field command in the Second Boer War (1899–1902), on which he published a book of lessons learned in 1903.

However, further promotion was checked by his having come into conflict with his commander-in-chief, who regarded him as unsuited for senior command in part because of his writings; Pilcher was a keen student of the German army and its operational methods, and an active theorist who published a number of controversial books advocating the adoption of new military techniques as well as an anonymous invasion novel.

[3] He was the eldest of five children (four surviving infancy); his younger brother, Percy, would go on to become a pioneering aeronautical engineer,[3] assisted by their sister Ella who undertook the fabric work on the aircraft's wings.

[3] Thomas brought his three younger siblings home, and entered the British Army, commissioned as a second lieutenant in the Dublin City Artillery Militia in August 1878.

[8] After a period of regimental service, Pilcher attended the Staff College, Camberley, passing the course in 1892, and from 1895 to 1897 was appointed as the deputy assistant adjutant general (DAAG) for Dublin District.

[14] For his services during the war, he was appointed a Companion of the Order of the Bath (CB) on 29 November 1900,[15] but was not invested until he was back in England, by King Edward VII at Buckingham Palace on 8 August 1902.

[18] The elder son, Sir Gonne Pilcher ("Toby", to his family), became a High Court judge,[19] while Tommy would be killed at the Battle of Neuve Chapelle in 1915, aged 21.

[20] Pilcher's marriage was not a happy one; a gambler and womaniser, he expected his independently wealthy wife to bail out his debts and turn a blind eye to his mistresses.

[12] His writing was sometimes controversial, beginning with the 1896 Artillery from an Infantry Officer's Point of View, in which he argued strongly in favour of adopting indirect fire techniques from concealed locations.

[23] As well as provoking debate, his writing proved problematic for his career; in part because of a 1907 pamphlet, Fire Problems, he was twice blocked for promotion by the Commander-in-Chief India.

The Spectator was dismissive, comparing the novel unfavourably to The Invasion of 1910 ("many useful hints are given as to practical lessons ... [but] the general plot entirely destroys any value it might otherwise possess") and noting that the suggestions were "highly typical", but that it was counterproductive to simply malign the Army and encourage the country to adopt German military policies.

At the outbreak of the First World War in August 1914 Pilcher was on leave in England, and offered his services to the War Office, but was initially turned down as unsuitable for command by the commander-in-chief (C-in-C) of the British Expeditionary Force (BEF), Field Marshal Sir John French;[25] however, in January 1915, he was appointed as general officer commanding (GOC) of the newly formed 17th (Northern) Division, a New Army formation predominantly drawn from civilian volunteers from Northern England.

He ordered an attack with the minimal number of men necessary, assuming it would inevitably be doomed to failure and high casualties, which outraged his superiors.

[9] Pilcher later wrote that: It is very easy to sit a few miles in the rear and get credit for allowing men to be killed in an undertaking foredoomed to failure, but the part did not appeal to me and my protests against these useless attacks were not well received.