Wikipedia
The British Commanders'-in-Chief Mission to the Soviet Forces in Germany (BRIXMIS) was a military liaison mission which operated behind the Iron Curtain in East Germany during the Cold War.
BRIXMIS existed from 1946 – shortly after the end of the Second World War – until the eve of the reunification of Germany in 1990. Created by an agreement to exchange military missions, the stated object of BRIXMIS – and the Soviet equivalent in the British Zone, SOXMIS – was "to maintain Liaison between the Staff of the two Commanders-in-Chief and their Military Governments in the Zones".
This liaison was undertaken by 31 members – 11 officers and no more than 20 others – appointed to each mission. These liaison staff were issued passes allowing freedom of travel and circulation, with the exception of certain restricted areas, within each other's zone. Such "tours", as they became known, were conducted in uniform and in clearly identifiable vehicles. Nevertheless, although never openly stated, this liaison role also presented an ideal opportunity for the gathering of military intelligence through reconnaissance and surveillance and the occasional 'borrowing' of military matériel. This opportunity was fully exploited by both sides.
BRIXMIS was ideally placed to "test the temperature" of Soviet intentions from its privileged position behind the Iron Curtain. However, and perhaps more importantly, it offered a channel for communication between West and East via its secondary but significant role of liaison – the initial reason for its establishment.