Chapter 7 examines the causes of the Thirty Years‘ War through the lens of the Protestant Union and Catholic Liga. While each alliance’s smaller Estates successfully imposed their vision on the larger Estates in the early 1610s, internal conflict in the Union and Liga resurfaced almost a decade later at the start of the Thirty Years‘ War. The Union’s inability to resolve the internal differences that first crystallized in 1610 proved decisive in its defeat and dissolution in 1621. Conversely, the Liga’s ability to balance the visions of its leading territory, Bavaria, with the alliance’s smaller ecclesiastical states explains its military success. The Liga’s capacity for serving the interests of all members underscores how it supported Bavaria’s expansion as a territorial state, maintained the integrity of its smaller member states, and influenced the development of the Empire’s political system during the war. This chapter therefore reevaluates the causes and course of the Thirty Years‘ War through the politics of alliance. It shows that the religious and constitutional impulses driving the war were in many cases inseparable for the parties involved.