The other monarchies of Europe looked with concern upon the developments in France, and considered whether they should intervene, either in support of Louis or to take advantage of the chaos in France.
The key figure was Marie-Antoinette's brother, the Holy Roman Emperor Leopold II.
Initially, he had looked on the Revolution with equanimity.
However, he became more and more disturbed as it became more and more radical.
Despite this, he still hoped to avoid war.

On 27 August, Leopold and Frederick William II of Prussia, in consultation with émigrés French nobles, issued the Declaration of Pillnitz, which declared the interest of the monarchs of Europe in the well-being of Louis and his family, and threatened vague but severe consequences if anything should befall them.
Although Leopold saw the Pillnitz Declaration as an easy way to appear concerned about the developments in France without committing any soldiers or finances to change them, the revolutionary leaders in Paris viewed it fearfully as a dangerous foreign attempt to undermine France's sovereignty.

In addition to the ideological differences between France and the monarchical powers of Europe, there were continuing disputes over the status of Austrian estates in Alsace, and the concern of members of the National Constituent Assembly about the agitation of émigrés nobles abroad, especially in the Austrian Netherlands and the minor states of Germany.

In the end, the Legislative Assembly, supported by Louis XVI, declared war on Austria ("the King of Bohemia and Hungary") first, voting for war on 20 April 1792, after a long list of grievances was presented to it by the foreign minister, Charles François Dumouriez.
Dumouriez prepared an immediate invasion of the Austrian Netherlands, where he expected the local population to rise against Austrian rule.
However, the Revolution had thoroughly disorganised the army, and the forces raised were insufficient for the invasion.
The soldiers fled at the first sign of battle and, in one case, on 28 April 1792, murdered their general, Irish-born comte Théobald de Dillon, whom they accused of treason.

While the revolutionary government frantically raised fresh troops and reorganised its armies, a Prussian-Austrian army under Charles William Ferdinand, Duke of Brunswick assembled at Coblenz on the Rhine.
In July, the invasion began, with Brunswick's army easily taking the fortresses of Longwy and Verdun.
The duke then issued on 25 July a proclamation called the Brunswick Manifesto, written by Louis's émigré cousin, the Prince de Condé, declaring the intent of the Austrians and Prussians to restore the king to his full powers and to treat any person or town who opposed them as rebels to be condemned to death by martial law.

Contrary to its intended purpose of strengthening Louis XVI's position against the revolutionaries, the Brunswick Manifesto had the opposite effect of greatly undermining his already highly tenuous position.
It was taken by many to be the final proof of collusion between the king and foreign powers in a conspiracy against his own country.
The anger of the populace boiled over on 10 August when an armed mob – with the backing of a new municipal government of Paris that came to be known as the Insurrectional Paris Commune – marched upon and invaded the Tuileries Palace.
The royal family took shelter with the Legislative Assembly.
The other monarchies of Europe looked with concern upon the developments in France, and considered whether they should intervene, either in support of Louis or to take advantage of the chaos in France. The key figure was Marie-Antoinette's brother, the Holy Roman Emperor Leopold II. Initially, he had looked on the Revolution with equanimity. However, he became more and more disturbed as it became more and more radical. Despite this, he still hoped to avoid war.
On 27 August, Leopold and Frederick William II of Prussia, in consultation with émigrés French nobles, issued the Declaration of Pillnitz, which declared the interest of the monarchs of Europe in the well-being of Louis and his family, and threatened vague but severe consequences if anything should befall them. Although Leopold saw the Pillnitz Declaration as an easy way to appear concerned about the developments in France without committing any soldiers or finances to change them, the revolutionary leaders in Paris viewed it fearfully as a dangerous foreign attempt to undermine France's sovereignty.
In addition to the ideological differences between France and the monarchical powers of Europe, there were continuing disputes over the status of Austrian estates in Alsace, and the concern of members of the National Constituent Assembly about the agitation of émigrés nobles abroad, especially in the Austrian Netherlands and the minor states of Germany.
In the end, the Legislative Assembly, supported by Louis XVI, declared war on Austria ("the King of Bohemia and Hungary") first, voting for war on 20 April 1792, after a long list of grievances was presented to it by the foreign minister, Charles François Dumouriez. Dumouriez prepared an immediate invasion of the Austrian Netherlands, where he expected the local population to rise against Austrian rule. However, the Revolution had thoroughly disorganised the army, and the forces raised were insufficient for the invasion. The soldiers fled at the first sign of battle and, in one case, on 28 April 1792, murdered their general, Irish-born comte Théobald de Dillon, whom they accused of treason.
While the revolutionary government frantically raised fresh troops and reorganised its armies, a Prussian-Austrian army under Charles William Ferdinand, Duke of Brunswick assembled at Coblenz on the Rhine. In July, the invasion began, with Brunswick's army easily taking the fortresses of Longwy and Verdun. The duke then issued on 25 July a proclamation called the Brunswick Manifesto, written by Louis's émigré cousin, the Prince de Condé, declaring the intent of the Austrians and Prussians to restore the king to his full powers and to treat any person or town who opposed them as rebels to be condemned to death by martial law.
Contrary to its intended purpose of strengthening Louis XVI's position against the revolutionaries, the Brunswick Manifesto had the opposite effect of greatly undermining his already highly tenuous position. It was taken by many to be the final proof of collusion between the king and foreign powers in a conspiracy against his own country. The anger of the populace boiled over on 10 August when an armed mob – with the backing of a new municipal government of Paris that came to be known as the Insurrectional Paris Commune – marched upon and invaded the Tuileries Palace. The royal family took shelter with the Legislative Assembly.
Louis XVI