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. 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. Brooke was educated in Pau, France, where he lived until the age of 16: he was bi-lingual in French and English. He was also fluent in German, and had learnt Urdu and Persian.
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. During the First World War, he served with the Royal Artillery in France where he gained 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 machine gun fire. Brooke was with the Canadian Corps from early 1917 and planned the barrages for the Battle of Vimy Ridge having at his disposal the Corps artillery and that loaned from the British First Army. In 1918 he was appointed GSO1 as the senior artillery staff officer in the First Army. Brooke ended the conflict as a lieutenant colonel with the Distinguished Service Order and Bar.
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 co-operation 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.
Following the outbreak of the Second World War, in September 1939, Brooke commanded II Corps in the British Expeditionary Force (BEF)—which included in its subordinate formations the 3rd Infantry Division, commanded by the then Major General Bernard Montgomery, as well as Major General Dudley Johnson's 4th Infantry Division. As corps commander, Brooke had a pessimistic view of the Allies' chances of countering a German offensive. He was sceptical of the quality and determination of the French Army, and of the Belgian Army. This scepticism appeared to be justified when he was on a visit to some French front-line units; and was shocked to see unshaven men, ungroomed horses and dirty vehicles.
He had also little trust in Lord Gort, Commander-in-Chief (C-in-C) of the BEF, whom Brooke thought too much interested in details and incapable of taking a broad strategic view. Gort, on the other hand, regarded Brooke as a pessimist who failed to spread confidence, and was thinking of replacing him.