Although born in London (on 11 December 1893) William George Stevens had deep New Zealand roots: his grandfather was an early settler of Southland, and his father, William Stevens, was an employee of the New Zealand Shipping Company. His mother, Catherine Sclanders, was a teacher in London prior to her marriage.
The family emigrated to New Zealand in early 1895. As a master mariner, Stevens's father was often away from home and his mother was the chief influence on him during childhood. He attended Beresford Street School and Auckland Grammar School. Academically gifted, he was headed for a career in civil engineering until his mother pushed him into seeking a cadetship at the newly established Royal Military College of Australia at Duntroon, Canberra.
Although he passed the entrance examination and was selected to attend in 1912, he was prevented from entering the college when a last-minute medical examination revealed a heart murmur. He eventually did so six months later after his parents insisted on a re-examination. Stevens found life at Duntroon agreeable and his ability was soon recognised.
With the outbreak of the First World War his class was graduated early, in November 1914. He was posted to the New Zealand Expeditionary Force (NZEF) and went overseas as a junior officer in the New Zealand Artillery in April 1915. He took part in the Gallipoli campaign from August until the evacuation in December, though he was absent, ill, at Malta during October.
Stevens went with the New Zealand Division to France in April 1916, and entered the line near Armentières the following month. A riding accident in August left him with a crushed foot, and he was hospitalised in Britain. He took the opportunity to summon his fiancée, Gladys Caroline Dora Barker, from Auckland, and they were married at St John's Church, Chelsea, London, on 2 December 1916; they were to have three sons.
Returning to France early in 1917, Stevens was attached to the New Zealand Divisional Headquarters, but almost immediately he became ill and was evacuated to Britain again. After recuperating until July, he served in a battery before being attached, in October, to the New Zealand Divisional Artillery, two months later becoming its staff captain. He was promoted to major in February 1918.
Stevens's service with the NZEF ended on 1 September 1919, and he attended an artillery course before returning to New Zealand early in 1920 to resume his career as a captain in the regular force. He planned to leave the army, but changed his mind when the possibility of attending the Staff College at Camberley, England, arose. After topping the entrance examination, he went to Britain, now with the rank of major, to undertake the two-year course, after which he attended the Imperial Defence College, London. During this latter period he also served as military adviser to the New Zealand high commissioner, which merely entailed attending ceremonial and other functions. Convinced that security in the future would require greater co-ordination of all the state's resources, he returned to New Zealand in February 1929 keen to promote his views.
After commanding the artillery in the army's central command at Palmerston North and writing a brief (unpublished) history of the New Zealand military forces for staff college use, Stevens assumed command of the General Headquarters Training Depot at Trentham. From 1931, with one brief interruption, he held concurrently a staff position in GHQ's operations and intelligence branch ('a phantom branch', as he later termed it), and was involved in the first attempts at co-ordinating defence activities with other arms of government. He was an early member of the New Zealand Institute of International Affairs.
In early 1937 Stevens went to Australia as New Zealand's exchange officer during the Australian army's camping season. While there he was overjoyed to be appointed secretary of New Zealand's Council of Defence, which entailed being seconded to the Prime Minister's Department. He was also secretary of the Chiefs of Staff Committee. Described by Carl Berendsen as 'exceedingly intelligent, well informed and energetic', Stevens oversaw – at first single-handedly – a web of committees, collectively named the Organisation for National Security, whose task was to prepare the 'New Zealand Government War Book' and to plan New Zealand's war effort.
The value of Stevens's work was demonstrated by the smooth transition to a war footing when New Zealand declared war on Germany on 3 September 1939. In November he accompanied the acting prime minister, Peter Fraser, to London as military adviser with the temporary rank of colonel. He visited France with Fraser, and was present at the preliminaries to Bernard Freyberg's appointment as commander of New Zealand's expeditionary force.
Stevens's administrative talents were required for the Second New Zealand Expeditionary Force (2NZEF). He left New Zealand as assistant adjutant and quartermaster general in the New Zealand Division's headquarters, but was involved in the administration of 2NZEF as a whole. The workload was so heavy that he relinquished his divisional responsibilities and was appointed, on 1 October 1940, as officer in charge of administration in 2NZEF's headquarters, a post he held for the rest of the war. From August 1942 until the headquarters transferred to Italy in early 1944, he also commanded Maadi Camp, in Cairo. Stevens was mentioned in dispatches, made a CBE, and promoted to brigadier in 1941; three years later he was made a CB and again mentioned in dispatches. He would later describe his time as Freyberg's staff officer as 'the greatest experience of my life'. His 2NZEF career was capped by his appointment to succeed Freyberg on 22 November 1945, and his promotion to major general. Stevens completed the disbandment of the force in February 1946. After visiting Jayforce in Japan, he returned to New Zealand the following month.
The prospect of resuming his career in the peacetime army had long filled Stevens 'with complete horror', though he confessed to some slight disappointment at not being selected as its first postwar commander. He had lobbied successfully to join the Department of External Affairs, which he did on 17 June 1946. Soon afterwards he was appointed official secretary of the New Zealand High Commission in London. Although he relished living in London again, he found the position uncongenial and difficult, and was disappointed that his diplomatic career was confined to an administrative role. He retired in June 1953, but continued to live in England, near the village of Much Hadham, Hertfordshire, until 1961. He was president of the New Zealand Society in 1958. Cost of living and family considerations then persuaded him to return to New Zealand, where he lived at Richmond.
In retirement Stevens wrote two of the official war histories, Problems of 2NZEF (which he described as 'largely a personal report') and Bardia to Enfidaville, as well as a short study of Freyberg. He also wrote his memoirs, which were unpublished. His wife died in 1967, and from November 1974 he lived in an old people's home in Stoke. He died of a heart attack in Nelson Hospital on 7 August 1975.
In 1920 one of Stevens's superiors had suggested in a confidential report that he was 'not really a soldier' and that he 'would have made an excellent Professor'. Stevens was essentially an outsider in the regular army – just as he was later, as an army officer, in the diplomatic service and even as an official historian. Blessed with high intelligence and administrative ability, he was able to reach the top of his profession despite the diffidence with which he entered it.