Until recently, scholars have assumed that Liberal Judaism's pre-war stance against Zionism was motivated primarily by a desire to assimilate into bourgeois English cultural mores. This article argues to the contrary: that the founders of Liberal Judaism were expressly trying to combat secular assimilation. Focusing on speeches and writings from Liberal Judaism's three primary founders, Lily Montagu, Claude Montefiore and Rabbi Israel Mattuck, I find they took a nuanced and principled approach to opposing Jewish nationalism. Their opposition to Zionism stemmed, instead, from a desire to contest definitions of Jewishness. In particular, they were concerned that national conceptions of Jewishness undermined their ethical and spiritual project. I conclude that many of their concerns anticipate problems in modern-day Israel, so that their arguments are worth revisiting.