The key figure in initial foreign reaction to the revolution is Holy Roman Emperor Leopold II, brother of Louis XVI's Queen Marie Antoinette.

Leopold had initially looked on the Revolution with equanimity, but has become more and more disturbed as the Revolution becomes more radical, although he still hopes to avoid war.

On August 27, Leopold and King Frederick William II of Prussia, in consultation with emigrant French nobles, had 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 sees the Pillnitz Declaration as a non-committal gesture to placate the sentiments of French monarchists and nobles, it is seen in France as a serious threat and is denounced by the revolutionary leaders.

France eventually issues an ultimatum demanding that the Habsburg Monarchy of Austria under Leopold II, who also is Emperor of the Holy Roman Empire, renounce any hostile alliances and withdraw its troops from the French border.

The reply is evasive and the Assembly votes for war on April 20. 1792 against Francis II (who has succeeded Leopold II), after a long list of grievances presented by foreign minister Dumouriez.