 His biggest success would be the handling of La Dame Blanche, a group of more than a thousand Belgian and French agents who monitored the movement of German troop trains to and from the Western Front.Named after a mythical White Lady whose appearance was supposed to presage the downfall of the German Imperial House of Hohenzollern, it was arguably the most effective intelligence operation of the First World War and, according to Cumming, produced 70 per cent of all Allied intelligence on the German forces worldwide.After the war Landau was sent to lead the passport control office in Berlin, in theory a very prestigious post within MI6.