Between March 1965 and April 1966 1 RAR and its attached field battery were under command of the US 173rd Airborne Brigade and fought as a component part of that formation. It was whilst attached to 173rd Airborne Brigade that 1 RAR discovered the Cu Chi tunnel complex.
The arrangement wasn’t entirely satisfactory. The original rationale for Australian involvement was defence of the Bien Hoa airbase and that limited their deployment radius. Added to that, the Australians had a vastly different approach to counter insurgency warfare and were not prepared to be thrown into the sort of meat grinder ops favoured by their allies.
Therefore when 1 RAR were replaced by the expanded 1ATF it was decided to give them their own operational patch, Phuoc Tuy Province, where they stayed for pretty much the rest of the war. There were US troops attached to 1 ATF, most notably a 155mm SP battery and 1ATF occasionally worked with US formations as part of larger II Field Force operations ,notably during Tet and The May 68 Mini Tet, these were limited and usually in areas contiguous to Phuoc Tuy.
The US 11th Armoured Cavalry was based to the north of Phuoc Tuy and occasionally operated into the province and in support of 1ATF operations.
On the whole 1ATF operated on its own, albeit with US air support and often considerable air lift and logistic support. The average digger was quite happy with this arrangement as the tin tanks were a bloody disaster in the field and their concept of operations quite alien to Australian methods.