On December 10, 1757 R. David Fränckel (1707–1762), Chief Rabbi of Berlin Jewry, delivered in German a sermon on the occasion of Frederick the Great's victory at Leuthen. Scholarly consensus has ascribed this sermon to Moses Mendelssohn (1729–1796), and it is included in the authoritative edition of Mendelssohn's Complete Works (Jubiläumsausgabe). Drawing on an earlier sermon by Fränckel that has only recently come to light, this paper argues that the "Leuthen Sermon" was in truth authored by Fränckel himself, in Hebrew, and that Mendelssohn only translated it into German. This re-attribution affords a better appreciation of Fränckel's important role in the emergence of the Berlin Haskalah. It is also suggested that Fränckel's thought was closer to Mendelssohn's than hitherto realized, and that Fränckel played a greater role in Mendelssohn's intellectual development than previously thought. The Appendix points out that Fränckel's sermon enjoyed a world-wide success: the German version was reprinted a considerable number of times in Germany; and an English translation was published in London and was reprinted in the New World by both Jews and Christians.