As ambassador and deputy head of mission to the United Nations from 1971 to 1978, he oversaw the Federal Republic of Germany regaining its status from observer to full member.

[1] Between 1933 and 1937, she noted her father‘s reaction to the rise of Nazism and the negative feelings he was unwilling to commit to paper, but which he confided to his family in a diary she kept.

After spending some time in Qingdao (then known as the German colony of "Tsingtao") and London, he began in 1909 to work as a graduate civil servant (Assessor) in the Foreign Office.

After the war ended in 1918, Hassell joined the nationalist German National People's Party (Deutschnationale Volkspartei or DNVP).

In 1938, as a result of the Blomberg-Fritsch Affair, Hassell was recalled from his posting as ambassador in Rome by Adolf Hitler but without being cast out of the diplomatic service.

[4] Over the next few years, Hassell used his position in the executive committee of the Central European Economic Congress to discuss with Allied officials what might happen after a possible coup d'état in Germany.

[citation needed] Depending on the source, either he or the former ambassador to the Soviet Union, Friedrich Werner von der Schulenburg, would have become Foreign Minister in the foreseen transitional government.

Hassell's offer in 1940 was based on the condition that Britain would let Germany keep almost all of the Nazi’s territorial gains in Europe, including Austria, Sudetenland and Poland.

On 8 September, after a two-day trial at the German People's Court (Volksgerichtshof), over which Roland Freisler presided, he was sentenced to death and executed the same day at Plötzensee Prison in Berlin.