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. At the beginning of his career in 1966, Fleischer published a comprehensive and profound critique on Menahem Zulai’s book, The paytanic school of Rav Saadia Gaon, wherein Zulai had brought together and summarized his research on Rabbi Saadia Gaon. According to Professor Yosef Tobi, Fleischer occasionally found that there exists a direct influence from Saadia upon Spanish works, such as the integration of philosophical ideas in the liturgical poems composed by Joseph ibn Abitur and Solomon ibn Gabirol, the structure of the Spanish azharot and keter malkhut of Ibn Gabirol, the assignment of biblical verse as a linguistic model in poetry and alternate rhyming (abab). However, in all these assertive statements, there is still some ambivalence and uncertainty, seeing that Fleischer is careful to point out that “the secular Hebrew poetry of Spain is not a continuation of pre-Spanish secular poetry, but rather a new formation in terms of its virtues and character: a faithful reflection, in principle, of Arabic secular poetry.”