A network of resistance fighters was formed across Britain and arms caches were established. The network was recruited, in part, from the 5th (Ski) Battalion of the Scots Guards (which had originally been formed, but was not deployed, to fight alongside Finnish forces fighting the Soviet invasion of Finland). The network, which became known as the Auxiliary Units, was headed by Major Colin Gubbins – an expert in guerrilla warfare (who would later lead SOE). The units were trained, in part, by "Mad Mike" Calvert, a Royal Engineers officer who specialised in demolition by explosives and covert raiding operations. To the extent that they were publicly visible, the Auxiliary Units were disguised as Home Guard units, under GHQ Home Forces. The network was allegedly disbanded in 1944; some of its members subsequently joined the Special Air Service and saw action in .