At the time, violations of airspace at the border between West and East Germany were common, with, on average, two aircraft a month belonging to members of the North Atlantic Treaty Organisation (NATO) crossing into Eastern airspace while a much larger number of Soviet planes crossed into that of West Germany. There were 38 violation of West Germany's air space by Soviet aircraft in a period of just four weeks between August and September 1961. Some of the violations were deliberate, to determine the opposite side's reaction, while others were by mistake, caused by the difficulty in determining the border line from the air.