During the final quarter of the 20th century, the democratic peace thesis — the idea that democracies do not fight each other — moved to the centre of scholarly debate throughout the Western world. Much of this work traces its origins to the European Enlightenment, focusing especially on Immanuel Kant. Yet this narrative ignores earlier 20th-century debates about the possibilities of global peace, and the role of democracy within them. In this article, I analyse some prominent, but now largely forgotten, strands of political thinking in the United States and Britain during the first half of the 20th century. They form an important part of the genealogy of the democratic peace thesis. I start by delineating four types of argument about peace that were popular in the 19th century: liberal-systemic, radical-liberal, socialist and republican. I then introduce two other modes of argument that circulated at the turn of the 20th century: the ‘war thesis’ (the idea that democracies are war-prone) and the ‘empire peace thesis’ (the argument that only imperial states are capable of assuring perpetual peace). I follow this with a discussion of racial utopianism — the claim that the unification of the Anglo-Saxons could eliminate war, securing peace and justice on earth. This white supremacist vision was a call for the racial pacification of the globe.