You wouldn’t expect the KGB to be responsible for the title of a new film about the Lithuanian resistance movement in the late 1940s and early 1950s, but that’s the way it is: “the invisible front” was what the Soviet security forces operating in Lithuania in the 1940s and 50s called the self-trained fighters they were up against, known, in all three Baltic countries, as “forest brothers”, who would swoop out of the unfamiliar woods and sink back, literally, into the ground, into their forest bunkers.