He married Aileen Robertson in 1926 (died 1993), with whom he had one son and one daughter.
In 1926, Slim was sent to the Indian Staff College at Quetta. On 5 June 1929, he was appointed a General Staff Officer, Second Grade On 1 January 1930, he was given the brevet rank of major, with formal promotion to this rank made on 19 May 1933. His performance at Staff College resulted in his appointment first to Army Headquarters India in Delhi and then to Staff College, Camberley in England (as a General Staff Officer, Second Grade), where he taught from 1934 to 1937. In 1938, he was promoted to lieutenant-colonel and given command of the 2nd Battalion, 7th Gurkha Rifles. In 1939 he was briefly given the temporary rank of brigadier as commander of his battalion. On 8 June 1939, he was promoted to colonel (again with temporary rank of brigadier) and appointed head of the Senior Officers' School at Belgaum, India.
In March 1942, Slim was given command of 1st Burma Corps, also known as BurCorps, consisting of the 17th Indian Infantry Division and 1st Burma Division). Slim was made acting lieutenant-general on 8 May 1942. The corps was under attack in Burma by the Japanese and, heavily outnumbered, he was soon forced to withdraw to India. On 28 October 1942, he was appointed a Commander of the Order of the British Empire (CBE).
He then took over XV Corps under the command of the Eastern Army. His command covered the coastal approaches from Burma to India, east of Chittagong. He had a series of disputes with Noel Irwin, commander of Eastern Army and, as a result, Irwin (although an army commander) took personal control of the initial advance by XV Corps into the Arakan Peninsula. The operations ended in disaster, during which Slim was restored to command of XV Corps, albeit too late to salvage the situation. General Irwin and Slim blamed each other for the result but in the end Irwin was removed from his command and Slim was promoted to command the new Fourteenth Army—formed from IV Corps (Imphal), XV Corps (Arakan) and XXXIII Corps (reserve)—later joined by XXXIV Corps. On 14 January 1943, Slim was awarded the Distinguished Service Order (DSO) for his actions in the Middle East during 1941.
He quickly got on with the task of training his new army to take the fight to the enemy. The basic premise was that off-road mobility was paramount: much heavy equipment was exchanged for mule- or air-transported equipment and motor transport was kept to a minimum and restricted to those vehicles that could cope with some of the worst combat terrain on Earth. The new doctrine dictated that if the Japanese had cut the lines of communication, then they too were surrounded. All units were to form defensive 'boxes', to be resupplied by air and assisted by integrated close air support and armour. The boxes were designed as an effective response to the tactics of infiltration practiced by the Japanese in the war. Slim also supported increased offensive patrolling, to encourage his soldiers to lose both their fear of the jungle and also their belief that Japanese soldiers were better jungle fighters.
At the start of 1944, Slim held the official rank of colonel with a war-time rank of major-general and the temporary rank of lieutenant-general. In January 1944, when the Second Arakan Offensive was met by a Japanese counter-offensive, the Indian 7th Infantry Division was quickly surrounded along with parts of the Indian 5th Infantry Division and the 81st (West Africa) Division. The 7th Indian Division's defence was based largely on the "Admin Box"—formed initially from drivers, cooks, suppliers, etc. They were supplied by air—negating the importance of their lost supply lines. The Japanese forces were able to defeat the offensive into Arakan, but they were unable to decisively defeat the allied forces or advance beyond the surrounded formations. While the Second Arakan Offensive ended in failure, it proved tactics that were very effective against the Japanese.
In early 1944, Slim was appointed a Companion of the Order of the Bath (CB). Later in 1944 the Japanese launched an invasion of India aimed at Imphal—hundreds of miles to the north. Slim airlifted two entire veteran divisions (5th & 7th Indian) from battle in the Arakan, straight into battle in the north. Desperate defensive actions were fought at places such as Imphal, Sangshak and Kohima, while the RAF and USAAF kept the forces supplied from the air. While the Japanese were able to advance and encircle the formations of 14th Army, they were unable to defeat those same forces or break out of the jungles along the Indian frontier. The Japanese advance stalled. The Japanese refused to give up even after the monsoon started and large parts of their army were wrecked by conducting operations in impossible conditions. As a result their units took unsupportable casualties and were finally forced, in July 1944, to retreat in total disorder, leaving behind many dead. On 8 August 1944, Slim was promoted to lieutenant-general, and, on 28 September 1944, he was made a Knight Commander of the Order of the Bath (KCB). He was also mentioned in despatches.
In 1945, Slim launched an offensive into Burma, with lines of supply stretching almost to breaking point across hundreds of miles of trackless jungle. He faced the same problems that the Japanese had faced in their failed 1944 offensive in the opposite direction. He made the supply of his armies the central issue in the plan of the campaign. The Irrawaddy River was crossed (with the longest Bailey bridge in the world at the time—most of which had been transported by mule and air) and the city of Meiktila was taken, followed by Mandalay. The Allies had reached the open plains of central Burma, sallying out and breaking Japanese attacking forces in isolation, maintaining the initiative at all times, backed up by air-land co-operation including resupply by air and close air support, performed by both RAF and USAAF units.
In combination with these attacks, Force 136 helped initiate a countrywide uprising of the Burmese people against the Japanese. In addition to fighting the allied advance south, the Japanese were faced with heavy attacks from behind their own lines. Toward the end of the campaign, the army raced south to capture Rangoon before the start of the monsoon. It was considered necessary to capture the port because of the length of the supply lines overland from India and the impossibility of supply by air or land during the monsoon. Rangoon was eventually taken by a combined attack from the land (Slim's army), the air (parachute operations south of the city) and a seaborne invasion. Also assisting in the capture of Rangoon was the Anti-Fascist People's Freedom League lead by Thakin Soe with Aung San (the future Prime Minister of Burma and father of Aung San Suu Kyi) as one of its military commanders.
As the Burma campaign came to an end Slim was informed in May by Oliver Leese, the commander of Allied Land Forces South-East Asia (ALFSEA) that he would not be commanding Fourteenth Army in the forthcoming invasion planned for Malaya but would take command of the new Twelfth Army being formed to mop up in Burma. Slim refused the appointment, saying he would prefer to retire. As the news spread Fourteenth Army fell into turmoil and Alan Brooke, the Chief of the Imperial General Staff, furious at not having been consulted by Leese, and Claude Auchinleck, the C-in-C India who was at the time in London, brought pressure to bear. The Supreme Allied Commander of the Southeast Asia Theatre, Louis Mountbatten was obliged to order Leese to undo the damage. On 1 July 1945, Slim was promoted to general and was informed that he was to succeed Leese as C-in-C ALFSEA. However, by the time he took up the post, having taken some leave, the war was at an end.
In September 1949, he was appointed to the Army Council. On 2 January 1950, he was made a Knight Grand Cross of the Order of the Bath (GCB) and later that year was made a Chief Commander of the Legion of Merit by the United States. On 1 November 1952, he relinquished the position of Chief of the Imperial General Staff and, on 10 December 1952, was made a Knight Grand Cross of the Order of St Michael and St George (GCMG) on his appointment as Governor-General of Australia.
On 2 January 1953, he was appointed a Knight of the Order of St. John (KStJ). On 8 May 1953, he took up the post of Governor-General of Australia. On 27 April 1954, he was appointed a Knight Grand Cross of the Royal Victorian Order (GCVO).
Slim was a popular choice for Governor-General since he was an authentic war hero who had fought alongside Australians at Gallipoli and in the Middle East. In 1954 he was able to welcome Queen Elizabeth II on the first visit by a reigning monarch to Australia. Slim's duties as Governor-General were entirely ceremonial and there were no controversies during his term. The Liberal leader Robert Menzies held office throughout Slim's time in Australia. His Official Secretary throughout his term was Murray Tyrrell.
In 1959, Slim retired and returned to Britain, where he published his memoirs, Unofficial History and Defeat into Victory. On 24 April 1959, he was appointed a Knight Companion of the Order of the Garter (KG). On 15 July 1960, he was created Viscount Slim, of Yarralumla in the Capital Territory of Australia and of Bishopston in the City and County of Bristol. After a successful further career on the boards of major UK companies, he was appointed Constable and Governor of Windsor Castle on 18 June 1964. He died in London on 14 December 1970.
"the burly man who came to talk to the assembled battalion … it was unforgettable. Slim was like that: the only man I've ever seen who had a force that came out of him. British soldiers don't love their commanders ... Fourteenth Army trusted Slim and thought of him as one of themselves, and perhaps his real secret was that the feeling was mutual".
Lieutenant General Sir John Kiszely has recommended Slim's memoirs (Defeat into Victory) describing Slim as "perhaps the Greatest Commander of the 20th Century" and commenting on Slim's "self-deprecating style Slim's 14th Army was composed of an amalgam of Indian (Hindu, Sikh and Muslim troops), British, African, and other troops; he was on the far end of a long logistical pipeline and generally had the oldest equipment of any Allied army. By all accounts, he was a superb logistician, imaginative in his tactics and operational concepts, and - unusually - very popular with his troops.
As a British commander on the Asian mainland, Slim's contribution to the U.S. war effort in the Pacific has often been undervalued. For three years, Slim's soldiers tied down tens of thousands of Japanese troops in Burma that could have been otherwise redeployed against U.S. forces in New Guinea, the Philippines, Iwo Jima and Okinawa.
Military historian Max Hastings:
"In contrast to almost every other outstanding commander of the war, Slim was a disarmingly normal human being, possessed of notable self-knowledge. He was without pretension, devoted to his wife, Aileen, their family and the Indian Army. His calm, robust style of leadership and concern for the interests of his men won the admiration of all who served under him ... His blunt honesty, lack of bombast and unwillingness to play courtier did him few favours in the corridors of power. Only his soldiers never wavered in their devotion".
The spirit of comradeship Slim created within 14th Army lived on after the war in the Burma Star Association, of which Slim was a co-founder and first President.
A statue to Slim is on Whitehall, outside the Ministry of Defence, was unveiled by Queen Elizabeth II in 1990. Designed by Ivor Roberts-Jones, the statue is one of three of British Second World War Field Marshals (the others being Alanbrooke and Montgomery).
Slim's papers were collected by his biographer, Ronald Lewin, and given to the Churchill Archives Centre by Slim's wife, Aileen, Viscountess Slim, and son, John Slim, 2nd Viscount Slim, and other donors, 1977-2001. Lewin's biography, entitled Slim: The Standardbearer was awarded the 1977 WH Smith Literary Award