This chapter introduces the intrusive regionalism trend and explains why it’s puzzling: It’s happening in the global South (where we expect states to be particular “jealous” of their sovereignty) and it’s uneven (it varies across regions in the global South). It then lays out the methodological approach of the book, which is comparative-historical analysis, and presents the book’s explanatory framework, which seeks to account for the uneven rise of intrusive regionalism in the global South. The theory section covers the changing ideational and institutional context at the global level; the role of macronationalism in creating openings for norm contestation and erosion; and two more proximate factors: regime type and economic performance. Chapter 1 concludes by outlining the plan for the book.