Having graduated in 1936, he took an assistant position with Oppermann, typing the professor's manuscripts and maintaining his library, which brought him into contact with the journal Annales d'histoire économique et sociale.
Slicher van Bath then worked for some time as an archivist in Gelderland, being able to avoid conscription as a former sufferer of polio,[2] and returned to Oppermann's service in 1941.
[2] The thesis was finished in 1944, the doctorate degree awarded half a year after the end of World War II in November 1945.
At the request of the Cambridge Economic History of Europe's editorial board, Slicher van Bath wrote The agrarian history of Western Europe, AD 500–1850, his magnum opus (initially published in Dutch as De agrarische geschiedenis van West-Europa (500–1850), 1960).
The history of this continent, rather than Annales-style regional studies of the Netherlands and Western Europe, would be his primary research topic until his 1975 retirement.