The fledgling RAAF faced several challenges to its continued existence in the 1920s and early 1930s, and Williams received much of the credit for maintaining its independence, but an adverse report on flying safety standards saw him dismissed from the position of CAS and seconded to the RAF prior to World War II.

The unit departed Australia in March 1916 without any aircraft; after arriving in Egypt it received B.E.2 fighters, a type deficient in speed and manoeuvrability, and which lacked forward-firing machine guns.

[8] Williams wrote that in combat with the German Fokkers, "our fighting in the air was of short duration but could mean a quick end",[9] and that when it came to bombing, he and his fellow pilots "depended mainly on luck".

[9] Williams and the other Australians were initially involved in isolated tasks around the Suez Canal, attached to Royal Flying Corps (RFC) units.

[12] On 21 April, Williams landed behind enemy lines to rescue downed comrade Lieutenant Adrian Cole, having the day before pressed home an attack on Turkish cavalry while under "intense anti-aircraft fire"; these two actions earned him the Distinguished Service Order, the citation for which reads: For conspicuous gallantry and devotion to duty.

[1][15] As a Dominion officer, Williams found that he was not permitted to "exercise powers of punishment over British personnel", leading to him being temporarily "granted a supplementary commission in the Royal Air Force".

[19][20] Augmented by a giant Handley Page bomber, his forces took part in the Battle of Armageddon, the final offensive in Palestine, where they inflicted "wholesale destruction" on Turkish columns.

[21][22] Of 40th Wing's actions at Wadi Fara on 21 September 1918, Williams wrote: "The Turkish Seventh Army ceased to exist and it must be noted that this was entirely the result of attack from the air."

He also sent Captain Ross Smith in the Handley Page, accompanied by two Bristol Fighters, to aid Major T. E. Lawrence's Arab army north of Amman when it was harassed by German aircraft operating from Deraa.

Twice mentioned in despatches, by the end of the war Williams had established himself, according to Air Force historian Alan Stephens, as "the AFC's rising star".

[18][26] The AAF was duly formed on 31 March 1921; Williams deliberately chose this day rather than 1 April, the founding date of the RAF three years earlier, "to prevent nasty people referring to us as 'April Fools'".

[30][31] He also started a program to second students from the Army and Navy, including graduates of the Royal Military College, Duntroon, to bolster officer numbers; candidates reaped by this scheme included future Air Force chiefs John McCauley, Frederick Scherger, Valston Hancock and Alister Murdoch, along with other senior identities such as Joe Hewitt and Frank Bladin.

Considered prescient in many ways, it treated World War I ally Japan as Australia's main military threat,[36] and advocated inter-service co-operation while maintaining that none of the armed forces was "purely auxiliary to another".

He had visited the Irvin Air Chute Company while in the US during 1924 and recommended purchase at the time, but a backlog of orders for the RAF meant that the Australian equipment took almost two years to arrive.

Flying Officer Ellis Wackett was assigned to instruct volunteers at RAAF Richmond, and made the country's first freefall descent from a military aircraft, an Airco DH.9, on 26 May.

Williams himself jumped over Point Cook on 5 August, having decided that it would set "a good example if, before issuing an order for the compulsory wearing of parachutes, I showed my own confidence in them ..." Though his descent took him perilously close to the base water tank ("I thought it would be a poor ending to drown there, or even to be pulled out dripping wet") and "too close to be comfortable to a 30,000 volt electric transmission line", he completed the exercise unscathed.

On 25 September 1926, with two crew members including Goble's pilot, Ivor McIntyre, Williams commenced a 10,000-mile (16,000 km) round trip from Point Cook to the Solomon Islands in a De Havilland DH.50A floatplane, to study the South Pacific region as a possible theatre of operations.

[42] Though seen partly as a "matter of prestige" brought on by contemporary newspaper reports that claimed "'certain Foreign Powers'" were planning such a journey, and also as a "reaction" by Williams to Goble's 1924 expedition,[43] it was notable as the first international flight undertaken by an RAAF plane and crew.

[41] Williams was appointed a Commander of the Order of the British Empire (CBE) in the 1927 King's Birthday Honours in recognition of the achievement,[44] and promoted to air commodore on 1 July the same year.

[1] As CAS, Williams had to contend with serious challenges to the RAAF's continued existence from the Army and Navy in 1929 and 1932, arising from the competing demands for defence funding during the Great Depression.

He played a personal part in the creation of the Commonwealth Aircraft Corporation in November 1936, headed up by former Squadron Leader Lawrence Wackett, late of the RAAF's Experimental Section.

[45] Williams made the first overseas flight in an aeroplane designed and built in Australia when he accompanied Squadron Leader Allan Walters and two aircrew aboard a Tugan Gannet to Singapore in February 1938.

[42] A series of mishaps with Hawker Demons at the end of 1937, which resulted in one pilot dying and four injured, subjected the Air Force to harsh public criticism.

[57] He was able to negotiate improved conditions for RAAF personnel in Europe, including full Australian pay scales as opposed to the lower RAF rates that were offered initially.

[1] In 1977, Williams published his memoirs, These Are Facts, described in 2001 as "immensely important if idiosyncratic ... the only substantial, worthwhile record of service ever written by an RAAF chief of staff".

[10][25][66] The epithet had earlier been applied to Eric Harrison, who had sole charge of Central Flying School after Henry Petre was posted to the Middle East in 1915, and was also a founding member of the RAAF.

[6] Between the wars he had continually striven for his service's status as a separate branch of the Australian armed forces, seeing off several challenges to its independence from Army and Navy interests.

[29][36] His input to debate in the 1930s around the "Singapore strategy" of dependence on the Royal Navy for the defence of the Pacific region has been criticised as limited, and as having "failed to demonstrate the validity of his claims for the central role of air power".

[69] Unique at the time among Commonwealth forces, the uniform was changed to an all-purpose middle blue suit in 1972 but following many complaints in the ensuing years reverted to Williams' original colour and style in 2000.

[73] In 2005, Williams' Australian Flying Corps wings, usually on display at the RAAF Museum in Point Cook, were carried into space and back on a shuttle flight by Australian-born astronaut Dr Andy Thomas.