 Siegmund had a sincere and deep affection for his mother who taught him to have a critical and inquisitive mind.In the period immediately before the Second World War he worked under cover for the Z Organisation, a highly secret offshoot of MI6/SIS, and reported impressively from Switzerland on his regular meetings with Hjalmar Schacht, then the president of the Nazi German Reichsbank and thus the most powerful German banker.He was forced to flee the National Socialist regime of Adolf Hitler and moved to the United Kingdom in 1934 where he co-founded S. G. Warburg & Co. in 1946 with Henry Grunfeld.