After World War II, Fleischer was active in the Bnei Akiva movement in Romania and was imprisoned for his Zionist activities. While in prison, he wrote an epic Hebrew poem, “Massa Gog,” in which he predicted the downfall of Communism. The poem was smuggled out of Romania and was published in Israel, under a pen name, where it caused somewhat of a “literary sensation”. Further books of poetry followed in Romania, all under a pen name.
In 1960, Fleischer emigrated to Israel, where he researched medieval Hebrew literature. He received a doctorate from the Hebrew University of Jerusalem, where he also subsequently lectured until 1997.
His published work, most of it on poetry and prayer, covers a wide range of ancient and medieval Jewish life from Andalusia and Amsterdam to Syria and Cairo. He also wrote extensively about Judaism’s encounter with Islam and Christianity.