Justus Lipsius (1547–1606) was among the most famed intellectuals in his time, but was largely forgotten during the Enlightenment. Intellectually, he stood at an important crossroads, his thought incorporating both late Renaissance traits and precursors of the early modern age. In this article I give a brief intellectual background to Lipsius's thought before concentrating on his thought regarding the lawful interaction between polities, with a focus on lawful government, dissimulation, war, and empire. I then detail the way in which Lipsian thought critically informed later theory and practice. It contained an eclectic mix of divine law, natural law, and positive human law, with some elements borrowed and popularized from earlier writers and others being more original. In the end, his work stands out both as an important inspiration for later theorists and practitioners, and as an example of the many idiosyncrasies and possible trajectories that early international law could have adopted.