In early March 1815, Austria, the United Kingdom, and France entered into a clandestine agreement to form a defensive alliance specifically targeting Prussia and Russia. The motivation behind this alliance was rooted in the instability that had been created in Europe following the Napoleonic Wars. As France was wrestling with the potential resurgence of powerful neighboring states, the three nations recognized the possibility of Prussian and Russian expansionist ambitions that could threaten their own territorial integrity and political influence.

This secretive pact was formulated against a backdrop of increasing tension, as the Congress of Vienna had recently convened to redraw the map of Europe and establish a balance of power following the defeat of Napoleon. The alliance allowed these three nations to coordinate their military resources and strategies without broadcasting their intentions to their rivals, which underscored the fragile nature of alliances in the post-Napoleonic order.

The implications of this unity extended beyond mere military strategy; it highlighted the complexities of European diplomacy during this tumultuous period and set the stage for future conflicts as each state sought to assert its interests amid shifting alliances. This move also illustrated a prevalent fear of potential domination by either Prussia or Russia, which was perceived as a direct challenge to the interests of the traditional powers in Europe.