This book analyses the history of international law to reveal the significant role utopianism has played in developing the international legal system. In fact, when pinpointing the legal system’s most accelerated phases of development, it becomes increasingly apparent how integral utopianism has been in dealing with the international community’s most troubled periods such as the World Wars. However, States have on numerous occasions undermined utopianism, leading to situations where individuals and communities have been vulnerable to modes of oppression such as war or repressive regimes. Thus, by examining the League of Nations and United Nations, this book seeks to show why utopianism continues to be a vital ingredient when the international community is seeking to ensure its loftiest and most ambitious goals such as maintaining international peace and security, and why for the sake of such utopian aspirations, the primary position States enjoy in international law requires reassessment.