The Brunswick Manifesto was a proclamation issued by Duke Charles of Brunswick, commander of the allied Prussian and Austrian forces, in July 1792. With the French Revolution in full swing and King Louis XVI under increasing pressure from revolutionary leaders, this manifesto was a direct response to the escalating fears of royalist sympathizers in Europe. Duke Charles sought to reassure French royalists that they would not be abandoned. The manifesto threatened punitive military action against Paris if the king or his family were harmed. It stated that the city would be subjected to devastation and its inhabitants would face dire consequences for any attacks on the monarchy.

The document was initially intended to intimidate the revolutionary government and rally support among royalists. However, it had the opposite effect, fueling revolutionary fervor and outrage among the populace in Paris. The manifesto became a rallying cry for those who opposed foreign intervention and intensified the resolve of revolutionary leaders to defend the republic. Key revolutionary figures began to prepare for the looming confrontation with foreign powers, convinced that they must protect their new republic at any cost.

The backlash following the issuance of the Brunswick Manifesto was rapid. Within days, Paris witnessed a surge of revolutionary sentiment culminating in the mobilization of revolutionary militias. Duke Brunswick underestimated the resolve of the citizens, who responded to the threats with calls for martial action against the advancing enemies of the revolution. The manifesto thus played a pivotal role in escalating the tensions that would lead to the September Massacres and increased violence in the streets of Paris, further entrenching the ideological divides between monarchy supporters and revolutionaries.