At the instigation of Frans van Kalken [fr], she turned to contemporary history and produced a thesis entitled L'origine et l'évolution du Vonckisme, which enabled her to obtain her doctorate in 1923,[3] after which she entered the Lycée Emile Max in Schaerbeek in July 1924.
[6] While working as a high school teacher, Tassier resumed her studies and, in November 1934,[2] successfully defended her agrégation thesis on Histoire de la Belgique sous l'occupation française en 1792 et 1793 (History of Belgium under the French occupation in 1792 and 1793),[3] which made her the first Belgian university agrégée.
Before World War II, her three major books on the Brabant Revolution, its premises and its aftermath were published, beginning with her thesis of 1923, L'origine et l'évolution du Vonckisme.
There was another compilation work in 1944, Idées et profils du XVIIIe siècle, which reprinted articles that had appeared before the war, a reworked part of her 1934 dissertation, and the introductory lesson of her course L'esprit public en Belgique de 1725 à 1789; the same year, L'Histoire de la guerre mondiale, for a world war museum and an office of contemporary documentation; in 1951, La Belgique et l'entrée en guerre des États-Unis (1914-1917); as well as a variety of other articles and contributions up to 1954.
[8] At the end of her life, she turned her interests towards the sixteenth century and had already announced in 1951 the publication of the book Les Pays-Bas contre Philippe II: Egmont, a work that her premature death did not allow her to realize.