Born at Bagnères de Bigorre to a prominent Northern Irish family, Alan Brooke was educated in France and at the Royal Military College, Woolwich. During World War I he served with the Royal Artillery in France, ending the conflict as a Lieutenant-Colonel. Between the wars he was a lecturer at Camberley Staff College and the Imperial Defence College, where he worked with most of the leading British officers of the Second World War.
Following the outbreak of World War II, Brooke commanded the II Corps of the British Expeditionary Force and played a leading role in the evacuation of Allied troops from Dunkirk. In July 1940 he was appointed to command United Kingdom Home Forces and in December 1941 was promoted Chief of the Imperial General Staff and Chairman of the Chiefs of Staff Committee, a post which he held until 1946.
In this role, Brooke served as the foremost military advisor to the Prime Minister, Winston Churchill, and to Britain's allies. He was offered command of British forces in the Middle East, which he declined, believing that that he ought to remain in Britain to prevent Churchill from leading the country into any foolhardy military adventures. He believed that the Prime Minister had offered him command of the Allied invasion of Western Europe and was bitterly disappointed to be passed over in favour of General Dwight D. Eisenhower.
Brooke was created Baron Alanbrooke of Brookeborough in 1945 and Viscount Alanbrooke in 1946.
The publication in 2001 of Alanbrooke's uncensored War Diaries attracted attention for their insight into the day-to-day running of the British war effort and their, at times, forthright criticism of Winston Churchill and other leading figures of the time.