Appointed a Companion of the Order of the Bath in 1966, McLachlan's final post before retiring in 1968 was as Air Member for Supply and Equipment.

[3] He was one of four cadets sponsored that year by the Royal Australian Air Force (RAAF), which did not at that stage have its own officer training college.

3 (Army Cooperation) Squadron, operating Hawker Demon fighters out of RAAF Station Richmond, New South Wales, on 4 December 1939.

[9][10] In its original army cooperation role supporting the Australian 6th Division in the North African Campaign, the squadron was equipped with obsolescent Gloster Gladiator biplane fighters and Westland Lysander observation aircraft.

[9] As part of his unit's work-up for operations, McLachlan organised training exercises with the 6th Division, as well as written exams to test his men's knowledge of army jargon and air-to-ground communications.

[9] Prior to converting to Hawker Hurricanes that month, the unit was credited with destroying twelve Italian aircraft for the loss of five Gladiators and two pilots; McLachlan shot down a Fiat CR.42 on 10 December 1940, the same action in which fellow squadron member and future ace Gordon Steege claimed his first "kill".

[13] McLachlan was awarded the Distinguished Flying Cross (DFC) for his "fine qualities as a fighter pilot" and "determined leadership" in the face of often "overwhelming numbers of enemy aircraft"; the citation was promulgated in the London Gazette on 11 February 1941 under the name "Ian Duncan MacLachlan".

9 Operational Group, the RAAF's "premier fighting unit" in the South West Pacific Area (SWPA), whose purpose was to act as a mobile strike force in support of advancing Allied troops.

[22][23] In March the Beauforts took part in the Battle of the Bismarck Sea, "the decisive aerial engagement" in the SWPA according to General Douglas MacArthur, though they were unable to score any hits against Japanese ships.

As well as providing local air defence, and fighter escort for Australian bombers, the Kittyhawks were armed with incendiary and general-purpose bombs so that they could engage in ground attack missions, a practice that had already been employed by Commonwealth forces in the Middle Eastern theatre.

9 Group, McLachlan told its former commander, Air Commodore Joe Hewitt, that the USAAF was "leaping ahead" of the RAAF, which was being left to "clean up the remnants" of Japanese resistance.

Barely a year later, morale among senior RAAF fighter pilots had dropped to such an extent that eight of them tried to resign their commissions in the so-called "Morotai Mutiny".

[31][32] Mentioned in despatches on 9 March 1945 for his "gallant and distinguished service",[33][34] McLachlan returned to action in the South-West Pacific as commander of No.

It fought in Operation Oboe Six, the invasion of Labuan, from June and was based on the island when the Pacific War ended in August 1945.

[3] As AOC Training Command, McLachlan undertook two reviews that would have, according to the official history of the post-war RAAF, "a significant effect on the Air Force of the 1960s".

This led to a policy of cadets undertaking academic degrees, in line with similar institutions in the other armed services; the college was subsequently renamed RAAF Academy.

According to Air Force historian Alan Stephens, "even for such a shrewd and sardonic man as McLachlan, that was to prove a painfully prescient observation", as the new bomber was delivered six years late and massively over budget.

[49] He was appointed a Companion of the Order of the Bath in the 1966 New Year's Honours, the citation noting particularly his chairmanship of the two "historic" committees that reorganised RAAF College and the Air Force's command structure in the late 1950s.

[52] McLachlan completed his term as Air Member for Supply and Equipment on 23 July 1968 and retired from the RAAF; he was divorced from his wife the same year.

[1] In retirement he continued to exercise his interest in Australia's defence, joining in 1975 a group of pundits, including former Chief of the Air Staff Sir Alister Murdoch, who promoted the addition of nuclear weapons to the country's arsenal.