The invasion of Ukraine by the Russian Federation and the subsequent military campaigns entail several classical aspects of armed conflict. First, it is a type of international armed conflict between two sovereign states that had been prevalent until the middle of the twentieth century but not in the last several decades. It is also a direct intervention by a superpower into a neighboring state with the former’s aspiration of territorial expansion. This action evokes a scheme of war reminiscent of the nineteenth or early twentieth century. At the same time, however, the invasion is generating in the international community a sense of new phenomena, leading to a new era that may be different from the past three decades following the end of the Cold War. In fact, the hostilities between the Russian Federation and Ukraine, as well as reactions by other states and international organizations, have raised legal and political issues that require scholars to reexamine existing frameworks of the international community and individual rules of international law.