Alan Brooke was born in 1883 at Bagnères-de-Bigorre, Hautes-Pyrénées, to a prominent Anglo-Irish family from West Ulster with a long military tradition.[15] He was the seventh and youngest child of Sir Victor Brooke, 3rd Baronet, of Colebrooke, Brookeborough, County Fermanagh, Ireland, and the former Alice Bellingham, second daughter of Sir Alan Bellingham, 3rd Baronet, of Castle Bellingham in County Louth.[16] Brooke was educated in Pau, France, where he lived until the age of 16. Thanks to his upbringing in the country he became a fluent French speaker.[17]
After graduation from the Royal Military Academy at Woolwich Brooke was, on 24 December 1902, commissioned into the Royal Regiment of Artillery as a Second Lieutenant.[18] During World War I he served with the Royal Artillery in France where he got a reputation as an outstanding planner of operations. At the battle of the Somme in 1916 he introduced the French "creeping barrage" system, thereby helping the protection of the advancing infantry from enemy machinegun fire.[19] Brooke ended the conflict as a Lieutenant-Colonel with two DSOs.
Between the wars he was a lecturer at the Staff College, Camberley and the Imperial Defence College, where Brooke knew most of those who became leading British commanders of the Second World War. From the mid 1930s Brooke held a number of important appointments: Inspector of Artillery, Director of Military Training and then GOC of the Mobile Division. In 1938, on promotion to lieutenant-general he took command of the Anti-Aircraft Corps (renamed Anti-Aircraft Command in April 1939) and built a strong relationship with Air Marshal Hugh Dowding, the AOC-in-C of Fighter Command which laid a vital basis of cooperation between the two arms during the Battle of Britain. In July 1939 Brooke moved to command Southern Command. By the outbreak of the Second World War Brooke was already seen as one of the army's foremost generals.[20]

Sir Victor Alexander Brooke, 3rd Baronet (5 January 1843 – 27 November 1891), was an Anglo-Irish naturalist and baronet. He was the father of Field Marshal Sir Alan Brooke, 1st Viscount Alanbrooke, and grandfather of Sir Basil Brooke, 1st Viscount Brookeborough, third Prime Minister of Northern Ireland