The princes of Europe, who had hitherto had no enemies but themselves, viewed it in the light of a common foe. The ancient relations of war and of alliance, already overlooked during the Seven Years' War, now ceased entirely: Sweden united with Russia, and Prussia with Austria. There was nothing now but the kings on one side, and people on the other, waiting for the auxiliaries which its example, or the faults of princes might give it. A general coalition was soon formed against the French revolution. Austria engaged in it with the hope of aggrandizement, England to avenge the American war, and to preserve itself from the spirit of the revolution; Prussia to strengthen the threatened absolute power, and profitably to engage its unemployed army; the German states to restore feudal rights to some of their members who had been deprived of them, by the abolition of the old regime in Alsace; the king of Sweden, who had constituted himself the champion of arbitrary power, to re-establish it in France, as he had just done in his own country; Russia, that it might execute without trouble the partition of Poland, while the attention of Europe was directed elsewhere; finally, all the sovereigns of the house of Bourbon, from the interest of power and family attachments. The emigrants encouraged them in these projects, and excited them to invasion. According to them, France was without an army, or at least without leaders, destitute of money, given up to disorder, weary of the assembly, disposed to the ancient regime, and without either the means or the inclination to defend itself. They flocked in crowds to take a share in the promised short campaign, and formed into organized bodies under the prince de Conde, at Worms, and the count d'Artois, at Coblentz.

The count d'Artois especially hastened the determination of the cabinets. The emperor Leopold was in Italy, and the count repaired to him, with Calonne as minister, and the count Alphonse de Durfort, who had been his mediator with the court of the Tuileries, and who had brought him the king's authority to treat with Leopold. The conference took place at Mantua, and the count de Durfort returned, and delivered to Louis XVI. in the name of the emperor, a secret declaration, in which was announced to him the speedy assistance of the coalition. Austria was to advance thirty- five thousand men on the frontier of Flanders; the German states, fifteen thousand on Alsace; the Swiss, fifteen thousand on the Lyonese frontier; the king of Sardinia, fifteen thousand on that of Dauphine; Spain was to augment its army in Catalonia to twenty thousand; Prussia was well disposed in favour of the coalition, and the king of England was to take part in it as elector of Hanover. All these troops were to move at the same time, at the end of July; the house of Bourbon was then to make a protest, and the powers were to publish a manifesto; until then, however, it was essential to keep the design secret, to avoid all partial insurrection, and to make no attempt at flight. Such was the result of the conferences at Mantua on the 20th May, 1791.

Louis XVI., either from a desire not to place himself entirely at the mercy of foreign powers, or dreading the ascendency which the count d'Artois, should he return at the head of the victorious emigrants, would assume over the government he had established, preferred restoring the government alone. In general Bouille he had a devoted and skilful partisan, who at the same time condemned both emigration and the assembly, and promised him refuge and support in his army. For some time past, a secret correspondence had taken place between him and the king. Bouille prepared everything to receive him. He established a camp at Montmedy, under the pretext of a movement of hostile troops on the frontier; he placed detachments on the route the king was to take, to serve him for escort, and as a motive was necessary for these arrangements, he alleged that of protecting the money despatched for the payment of the troops.

The royal family on its side made every preparation for departure; very few persons were informed of it, and no measures betrayed it. Louis XVI. and the queen, on the contrary, pursued a line of conduct calculated to silence suspicion; and on the night of the 20th of June, they issued at the appointed hour from the chateau, one by one, in disguise. In this way they eluded the vigilance of the guard, reached the Boulevard, where a carriage awaited them, and took the road to Chalons and Montmedy.