States and their international relations since 1816: introducing version 2 of the International System(s) Dataset (ISD)

We introduce version 2 of the International System(s) Dataset (ISD), a register of sovereign states across the 1816–2016 period that include numerous states that are missed in commonly used datasets like the Correlates of War (COW) Project. Whereas ISD version 1 identified 363 states between 1816 and 2011, version 2 identifies 482. This version also records valuable information on a range of corollary variables, including start dates, end dates, estimated population sizes, diplomatic relations with Europe, conflict episodes, the existence of borders, and the location of capital cities. This dataset makes an important contribution to the study of international relations. It provides a more accurate understanding of the development of the international system over the last two centuries, it moves beyond the Eurocentric bias that sits at the heart of existing quantitative IR scholarship, and it will enable scholars to pursue a range of research topics such as the historical importance of state borders and boundaries, the practices surrounding recognition, and the frequency and intensity of conflict across regions. In this article, we discuss the existing state system membership lists and show how the ISD addresses their shortcomings. We outline the key concept and operationalization of statehood that the ISD adopts. We detail the variables included in this version of the ISD, discuss the data collection process, and show temporal and spatial distributions that illustrate the uniqueness of the ISD. Finally, we demonstrate the utility of bringing the ISD into one of many potential research topics: the study of conflict.