'Stepsister' was a British and Australian programme to gather and move combat-experienced Australian forces, in the form of Major General Edmund F. Herring’s 6th Division and Major General Arthur S. Allen’s 7th Division followed later by Major General Leslie J. Morshead’s 9th Division, from the Middle East to the Far East and Australia (January/April 1942).
When Japan entered World War II with her 'Ai' attack on Pearl Harbor and a number of other offensives to take Hong Kong, Malaya and the Philippines, John Curtin, the prime minister of Australia was highly concerned that Australia was wide open to a Japanese invasion as the country possessed only minimal defences as the 2nd Australian Imperial Force had been sent to the Middle East and no British fleet was available to come to Australia’s aid. Curtin therefore requested the return of the Australian divisions from the Middle East so that they would be available for their own nation’s defence, and Winston Churchill ordered the Australian 6th and 7th Divisions to the Far East for the war against Japan.
The troops were to be returned in 'Stepsister', whereby 64,000 Australian troops and their equipment would be transported in 70 ships of varying sizes across the Indian Ocean, the exact destination undetermined. With German raiders and Japanese submarines operating in these waters, it is little wonder that Curtin got little sleep while the troops made their journey between January and April 1942.
Churchill originally wished the destination of the Australian formations to be Singapore. The British and commonwealth forces in Malaya outnumbered the Japanese, but were nonetheless forced to retreat toward the island Singapore in the face of the 'E' (i) advance of Lieutenant General Tomoyuki’s Yamashita’s 25th Army, which enjoyed air supremacy. Two major British warships, the modern battleship Prince of Wales and older battle-cruiser Repulse, which Churchill had sent to Singapore on 2 December 1941 at Curtin’s request for more forces for the defence of the island, but were sunk by Japanese warplanes on 10 December 1941 in their foray to locate and attack the Japanese invasion force.
By January 1942, Churchill realised that Singapore had little strategic value, but still wanted the besieged fortress to be reinforced as a Far Eastern bastion, and intended the island as the destination of the troops being transported in 'Stepsister'. Fortunately for the Allies in general and Australia in particular, the Australian troops from the Middle East did not arrive, as with its drinking water obtained from the Malay peninsula and little on the way of any naval or landward defences, Singapore could not and indeed did not resist the Japanese siege for long, and fell on 15 February 1942.
Churchill decided that the Australian forces would be better deployed under General Sir Archibald Wavell’s command in Java and Sumatra, but encountered strong Australian opposition as the two parties saw matters in different national and strategic lights. The UK and Australia, on opposite sides of the world, were each presented with different threats in their region. The UK faced Germany with the narrow English Channel separating them, while Australia faced Japan with the vastness of the Pacific Ocean and many guarded territories between them. Japan could do little to threaten the UK directly, and similarly Germany was only a threat to Australia if the UK was defeated, so Australia had sent three divisions to fight under British leadership in the war against Germany and Italy, and it was these whose return Churchill demanded for use in the war against Japan. The formations were about to be deployed under Wavell for the second time, for they had previously been under his command on their arrival in the Middle East, but now it was to be in Java under the ABDA command.
This prompted a five-day cable argument between Curtin and Churchill. The Australian defence chiefs advised that in light of the rapid Japanese advance, the divisions' delivery into South-East Asia made strategic nonsense, for the divisions would probably be destroyed or captured, in which case they could not defend Australia. It would be best, the Australians argued, that the divisions be despatched straight to Australia. On 15 February, Curtin took their advice and cabled Churchill, requesting that the ships still in transit be sent to Australia. It was the right choice, for on the following day Wavell informed the British government that Java could not be held, and therefore suggested that the Australian divisions should be diverted once more, in this instance to Burma to boost the defence against the Japanese 'B' (iii), and more specifically the Japanese assault on Rangoon. Churchill was predisposed to favour this idea, for if the Japanese took Burma, they could then invade India, and would control the Indian Ocean, which would mean Axis domination of both the Mediterranean Sea and the Indian Ocean, regions which encompassed many British interests.
On 17 February, after receiving news of the British plans regarding Burma, Curtin cabled Churchill again requesting that the troops be sent directly to Australia. Churchill was sure that if he could stall Curtin, he could change his Australian counterpart’s mind, so he ordered the diversion of the troopships toward Burma and, at Churchill’s request, President Franklin D. Roosevelt cabled Curtin, requesting that the divisions be sent to Burma. Churchill had convinced Roosevelt that it was necessary to keep the Burma Road open to supply China, and only the Australian divisions were close enough to do it.
Curtin refused to change his mind, meeting with the cabinet to say that it was now impractical to give 'substantial aid to countries outside Australia'. The war cabinet agreed to take steps for a full mobilisation of resources for the war and the Australian troops were committed to a return to Australia in the 'SU' series of convoys.