One result of the upheavals during the war year of 1866 in Germany was the conclusion of military alliances between the southern German states and the victorious Prussia. This study presents the four-year history of these so-called protective and defensive alliances—from their formation after the Austro-Prussian brother war in 1866 to their activation, the casus foederis, in the Franco-German War of 1870. It aims to contribute to the understanding of the prehistory of the founding of the Empire by examining a specific aspect of this period. Through the conflicts surrounding the protective and defensive alliances, particularly in southern Germany, the complexity and historical openness of the eventful years of the Empire's founding period become evident.