From 1864 to 1871, Prussia mounted a series of wars that fundamentally altered the balance of power in Europe. Yet no coalition emerged to check Prussia’s rise. Rather than balance against Prussian expansion, the great powers sat on the sidelines and allowed the transformation of European politics. Traditionally, scholars have emphasized structural variables, such as mulitpolarity, or domestic politics as the cause of this “underbalancing.” It was Prussia’s legitimation strategies, however—the way Prussia justified its expansion —that undermined a potential balancing coalition. As Prussia expanded, it appealed to shared rules and norms, strategically choosing rhetoric that would resonate with each of the great powers. These legitimation strategies undermined balancing coalitions through three mechanisms: by signaling constraint, laying rhetorical traps (i.e., framing territorial expansion in a way that deprived others states grounds on which to resist), and increasing ontological security (i.e., demonstrating its need to secure its identity in international politics), Prussia effectively expanded without opposition. An analysis of Prussia’s expansion in 1864 demonstrates how legitimation strategies prevented the creation of a balancing coalition.