From: Iron Kingdom: The Rise and Downfall of Prussia, 1600-1947, by Christopher Clark (Penguin, 2007), p. 285 (Kindle Loc. 5546-5584):
Tensions between the two German rivals had risen steadily during the 1780s. In 1785, Frederick II had taken charge of a coalition of German princes opposed to the annexation of Bavaria by the Habsburg Emperor Joseph II. In 1788, the Emperor had gone to war against the Turks, prompting fears that massive Habsburg acquisitions in the Balkans would give Austria the upper hand over her Prussian rival. But in the summer and autumn of 1789, as Austrian forces pushed back the armies of Sultan Selim III, a chain of revolts broke out across the peripheral territories of the Habsburg crown – Belgium, Tyrol, Galicia, Lombardy and Hungary. Frederick William II, a vain and impulsive man who was determined to live up to the reputation of his illustrious uncle, did his best to exploit the discomfort of the Austrians. The Belgians were encouraged to secede from Habsburg rule and the Hungarian dissidents were urged to rise up against Vienna – there was even talk of an independent Hungarian monarchy to be ruled by a Prussian prince.
Seen against this background, the revolution in France was welcome news, for there was good reason to hope that a new, ‘revolutionary’ French administration would put an end to the Franco-Austrian alliance. As the Prussians well knew, the alliance – along with its dynastic personification, Queen Marie Antoinette – was deeply unpopular with the Austrophobe patriots of the revolutionary movement. Berlin therefore courted the various revolutionary parties in the hope of building an anti-Habsburg ‘party’ in Paris. The aim was to reverse the diplomatic realignment of 1756, isolate Austria, and put an end to the expansionist plans of Joseph II. When a fully fledged revolution broke out in the prince-bishopric of Liège, a strip of territory right in the middle of Belgium, the Prussians supported the rebels there too, in the hope that the upheaval would spread to the adjacent Austrian-controlled areas.
There was also an ideological dimension to this tentative support for revolutionary upheaval. In 1789, a number of the leading Prussian policy-makers, including the minister responsible for foreign affairs, Count Hertzberg – were personally sympathetic to the aspirations of the revolutionaries. Hertzberg was a man of the enlightenment who deplored the incompetent despotism of the Bourbons in France. He saw Prussian support for the insurrection in Liège as entirely in keeping with the kingdom’s ‘liberal principles’. The envoy entrusted with handling Prussia’s affairs in the prince-bishopric, Christian Wilhelm von Dohm, was an enlightened official and intellectual (not to mention author of the famous tract supporting the emancipation of the Jews); he was a critic of the episcopal regime in Liège and favoured a progressive, constitutional solution to the dispute between the prince-bishop and the insurrectionists of the Third Estate.
It was above all the threat of a Prussian-backed revolution in Hungary that persuaded Joseph’s successor, Leopold II, to seek an understanding with Prussia. Leopold, a wise and temperate figure, saw at once the folly of pursuing new conquests in the Ottoman Balkans while his hereditary possessions disintegrated behind his back. In March 1790, he despatched a friendly letter to Berlin, opening the door for the negotiations that culminated in the Convention of Reichenbach of 27 July 1790. The two German powers agreed – after tense discussions – to pull back from the brink of war and put their differences behind them. The Austrians undertook to end their costly Turkish war on moderate terms (i.e. without annexations) and the Prussians promised to stop fomenting rebellions within the Habsburg monarchy.
The Convention looked innocuous, but it was more significant than it seemed. The era of bitter Prusso-Austrian antagonism that had structured the political affairs of the Holy Roman Empire since the invasion of Silesia in 1740 was now over, at least for a time, and the two German powers could pursue their interests in concert, rather than at each other’s expense. Following an oscillatory pattern that recalled the days of the Great Elector, Frederick William II abandoned his secret efforts to secure an alliance with Paris and switched to a policy of war against revolutionary France. Foreign Minister Hertzberg and his liberal views fell into disfavour; he was later dismissed. An important role in the new diplomacy went to Frederick William’s trusted adviser and confidant, Johann Rudolf von Bischoffwerder, an exponent of war against the revolution, who was despatched to Vienna in February and June–July 1791. The resulting Vienna Convention of 25 July 1791 laid the foundations for an Austro-Prussian alliance.
The first fruit of the Austro-Prussian rapprochement was a remarkable piece of gesture politics. The Declaration of Pillnitz, issued jointly by the Austrian Emperor and the Prussian king on 27 August 1791, was not a plan of action as such, but rather a statement of principled opposition to the Revolution.