I Corps was an Australian Army corps, one of three that were raised by the Army during World War II. It was the main Australian operational corps for much of the war. Various Australian and other Allied divisions came under its control at different times. In 1940–1942, the corps was based in the Mediterranean and Middle Eastern theatres, and controlled forces in action against the Germans, Italians and later the Vichy French in North Africa, Greece and Syria–Lebanon.

In 1942, following Japan's entry into the war, I Corps was transferred to the South West Pacific Area. Forces assigned to the corps undertook garrison duties in Ceylon, and briefly deployed to the ill-fated defence of Java in 1942, before returning to Australia. Between late 1942 and 1945, the corps oversaw Allied frontline units fighting against the Japanese in New Guinea and then Borneo in 1945.

At the outbreak of the Second World War, the Australian government decided to raise the Second Australian Imperial Force, an all volunteer force for overseas service that was separate to the previously existing part-time Militia. Initially, this force consisted of a single division – the 6th Infantry Division – as well as some base and support troops. On 25 February 1940, the decision was made to expand the 2nd AIF to a corps, consisting of two infantry divisions – the 6th and 7th – and throughout March and into April work began to start forming the new units, and establish a corps headquarters, in Melbourne. This formation would be responsible for the AIF units forming in Australia. On 11 April 1940, the establishment of I Corps was gazetted, and by the middle of the month the headquarters' strength was reported as 12 officers and 45 other ranks. Lieutenant General Thomas Blamey was appointed General Officer Commanding (GOC).

In late May 1940, approval was provided for the 2nd AIF to raise a third division – the 8th – although it would ultimately not serve with I Corps. Preparations also commenced for I Corps headquarters personnel to begin moving overseas. A small rear corps headquarters was to remain in Australia, although the responsibility for command of troops in Australia would be devolved to the 7th Division until it also deployed. The initial deployment of staff began in May, but the majority of draft made it only as far as Fremantle, Western Australia, before disembarking and returning to Melbourne, when the decision was made to divert this convoy to the United Kingdom, rather than the main corps-area in the Middle East. The personnel who reached the UK were subsequently used to form HQ Australforce. Meanwhile, the main element of the corps headquarters, including Blamey, reached Palestine in mid-June 1940. They arrived amidst fighting in the Western Desert, and it was considered that the AIF troops could soon be committed to battle.

Initially, when I Corps headquarters arrived there was only one Australian infantry division in the Middle East – the 6th, which was completing its training – however between October and December the 7th arrived, followed by the 9th by January 1941. During this time, the brigades assigned to each division were shuffled between superior headquarters as a result of several reorganisations to provide the better trained brigades to the formations likely to see combat first. At this time, the main elements of the corps headquarters moved forward towards the combat zone, leaving the administrative staff in Gaza and establishing themselves in Ikingi Maryut, in Egypt. Here they began preparations for operations. The 6th Infantry Division was the first to go into action, capturing Bardia in early January 1941. This was followed by further actions at Tobruk, Derna, Barce, and Benghazi, as the Italians were forced to withdraw across Libya by the Allies.

In February, I Corps took over control of Cyrenaica in Libya, replacing the British XIII Corps, and briefly adopting the designation of HQ Cyrenaica Force. This deployment lasted only a month, before I Corps headquarters was withdrawn back to Egypt to prepare for deployment to Greece for the campaign there, which began in April 1941. Initially, it had been planned for the 7th Division to deploy to Greece, but they were not considered fully trained, and as a result the 6th Division was dispatched. In their place, the newly arrived 9th Division replaced the 6th Division in the Western Desert, and they would subsequently take part in the Siege of Tobruk in mid- to late- 1941 when the Axis forces counter-attacked in the Western Desert.

Meanwhile, in Greece, the corps controlled the Australian 6th Division, the New Zealand 2nd Division, and the British 1st Armoured Brigade, as well as several ad hoc forces charged with the defence of northern Greece. On 12 April, it was officially renamed the Anzac Corps, a reference to the combined Australian-New Zealand formations of World War I. This was short-lived, though, as the Allied forces in Greece were quickly overcome by the German advance and after withdrawing from Gerania through Elasson, Larissa and Levadia, I Corps HQ left Greece on 23–24 April, and were subsequently evacuated to Egypt, although elements of the 6th Division were landed on Crete where they fought another short and unsuccessful campaign in May.

The corps re-formed in Deir Suneid, in Palestine, during which time the previous I Corps designation was assumed. In June 1941, as part of the initial plan for the invasion of Lebanon and Syria, held by Vichy French forces, I Corps was to take command of operations after Commonwealth forces reached their first objective, the Beirut–Damascus road. However, on 18 June, prior to that objective being reached, I Corps headquarters – based at Nazareth – took charge, to improve command and control of the Allied forces. From this date all Allied troops in the theatre came under the command of Lieutenant General John Lavarack, who took over from Blamey when he became deputy commander-in-chief in the Middle East. The forces under I Corps' command around this time included: 7th Division (less the 18th Infantry Brigade), the British 6th Infantry Division, the 1st (Free French) Light Division and the 5th Indian Infantry Brigade Group. The corps directed operations that captured Damascus and Damour before the armistice took effect on 12 July. Following the armistice in July, I Corps headquarters was established at Aley, near Beirut, and assumed responsibility for occupying all of Lebanon and Syria north of the Beirut–Damascus road. Eventually, the Australian 6th Infantry Division arrived to relieve the British 6th.