During World War I, in 1917, he was promoted to first lieutenant and married Maria Johanna van der Minne.

He was then recalled to The Hague, where he functioned as the right-hand man of General Reijnders in the role of head of the operations division.

Oster had been paired to Sas during the Olympic Games and both men liked each other beyond the regular courtesies and their families developed warm ties.

Oster had obtained an important position as the right hand of Wilhelm Canaris at the German Abwehr, the espionage and counter-espionage service of the army.

At 14:50 on that day Hitler gave the order for the invasion, but revoked it when he heard that England was not going to concede.

When he reported on 31 August that the war was now close, the head of the general staff intelligence section, Van der Plassche did not believe him, although others did.

During September and October 1939, Sas obtained more signals that the neutrality of the Netherlands would not remain inviolate.

This showed that the suspicions that Sas had were correct: the Netherlands would be invaded together with Belgium, albeit that the several German plans varied between the autumn of 1939 and February 1940 in relation to the Dutch component.

Sas had been promoted to lieutenant-general in a senior army function when he died in the 1948 KLM Constellation air disaster in Prestwick, Scotland.