British military historian and strategist known for his advocacy of mechanized warfare.

Liddell Hart left studies at Cambridge University when World War I broke out in 1914 and became an officer in the British Army. In 1920 he wrote the Army's official Infantry Training manual that included his “battle drill” system evolved in 1917 and his so-called “expanding torrent” method of attack, which grew out of infiltration tactics introduced in 1917–18. Liddell Hart became an early advocate of air power and mechanized tank warfare. Defining strategy as “the art of distributing military means to fulfil the ends of policy,” he favoured an “indirect approach” that aimed at dislocating the enemy and reducing his means of resistance. Drawing on his wartime experiences, he emphasized the elements of mobility and surprise.

Invalided in 1924, Liddell Hart retired as a captain in 1927. He was military correspondent of the Daily Telegraph in 1925–35 and military adviser to The Times in 1935–39. In 1937–38 he served as personal adviser to Leslie Hore-Belisha, secretary of state for war, and saw many of his advocated reforms implemented. His efforts to mechanize the Army with tank and antiaircraft forces were resisted by most professional officers.

Liddell Hart's writings were more influential in Germany than in France or England. His “expanding torrent” theory, along with the doctrines of General J.F.C. Fuller on employment of tanks, was adopted by German pioneers of armoured warfare, and became the basis for the blitzkrieg warfare through which German armies mastered the European continent in 1939–41. For the duration of the war, Liddell Hart wrote for the Daily Mail. Dubious of nuclear deterrence, he stressed conventional defense forces during the postwar years and also opposed the concept of total war. In 1966 he was knighted by Queen Elizabeth II.

Liddell Hart was the author of a number of military biographies, several works on military strategy, and a history of World War II.