Robertson returned to Australia in March 1946 to take over Southern Command.
In June he was appointed to replace Lieutenant General John Northcott as commander of the British Commonwealth Occupation Force (BCOF) in the occupation of Japan.
Northcott had negotiated the Northcott-MacArthur agreement in January 1946 with General of the Army Douglas MacArthur, which governed the terms and conditions under which the BCOF would occupy part of Japan.
They agreed that the BCOF would serve under American command, with American policy being followed.
It remained to make the occupation work.
Robertson had a poor relationship with the British component commander, Major General David Tennant Cowan, who resented being placed under an Australian officer. "
It did not occur to me", Robertson later wrote, "that officers of the British and Indian armies looked upon us from Australia and New Zealand as they looked upon Indians, and were prepared to do anything to avoid being publicly commanded by us."

The British government's principal interest in Japan was to renew pre-war trade concessions, and to secure new ones; it was particularly interested in the port city of Kobe.
The Americans blocked these attempts.
Robertson clashed repeatedly with Lieutenant General Sir Charles Gairdner, the official representative of the Prime Minister of the United Kingdom.
Gairdner was nominally senior in rank to Robertson until March 1947, when Robertson was promoted to the substantive rank of lieutenant general, backdated to January 1944.
Robertson resisted Gairdner's requests for BCOF aircraft for personal purposes.
Robertson returned to Australia in March 1946 to take over Southern Command. In June he was appointed to replace Lieutenant General John Northcott as commander of the British Commonwealth Occupation Force (BCOF) in the occupation of Japan. Northcott had negotiated the Northcott-MacArthur agreement in January 1946 with General of the Army Douglas MacArthur, which governed the terms and conditions under which the BCOF would occupy part of Japan. They agreed that the BCOF would serve under American command, with American policy being followed. It remained to make the occupation work. Robertson had a poor relationship with the British component commander, Major General David Tennant Cowan, who resented being placed under an Australian officer. "It did not occur to me", Robertson later wrote, "that officers of the British and Indian armies looked upon us from Australia and New Zealand as they looked upon Indians, and were prepared to do anything to avoid being publicly commanded by us."
The British government's principal interest in Japan was to renew pre-war trade concessions, and to secure new ones; it was particularly interested in the port city of Kobe. The Americans blocked these attempts. Robertson clashed repeatedly with Lieutenant General Sir Charles Gairdner, the official representative of the Prime Minister of the United Kingdom. Gairdner was nominally senior in rank to Robertson until March 1947, when Robertson was promoted to the substantive rank of lieutenant general, backdated to January 1944. Robertson resisted Gairdner's requests for BCOF aircraft for personal purposes.
Horace Robertson