The Unseen History of International Law locates and describes almost one thousand surviving copies of the first nine editions of Hugo Grotius' De iure belli ac pacis (IBP) published between 1625 and 1650. Meticulously reconstructing the publishing history of these first nine editions and cataloguing copies across hundreds of collections, The Unseen History provides fundamental data for reconstructing the impact of IBP across time and space. It also examines annotations that thousands of owners and readers have left in IBP copies over four centuries, offering original insights into the development of international law.Grotius' De iure belli ac pacis has been commonly regarded as the foundation of modern international law since its first appearance in 1625. Most major international law scholars have engaged with IBP, often owning and richly annotating their own copies. At key moments - including the demise of the Holy Roman Empire, the fall of Napoleon, and the end of both world wars - IBP was reissued with new commentaries by multinational projects devoted to restarting the international order. Despite the enormous literature on IBP's reception and influence, we cannot fully understand its impact without uncovering the history of IBP as a physical object, with hundreds of thousands of unpublished annotations arguing or agreeing with the text, updating and adapting its contents.Approaching Grotius' seminal work as a physical vehicle of the author's, the publishers', owners', and readers' engagement, The Unseen History radically expands and revises our understanding not only of IBP, but also of the academic discipline and lived practice of modern international law over the last four centuries. In addition to delving into the first nine editions' printing history, descriptive bibliography, and both Grotius' and the publishers' marketing and donation strategies, the book explores Grotius' subsequent impact on pro-slavery and abolitionist litigation as a case study of how the census' original findings can be applied to specific areas of reception.