The French Revolutionary Wars began on 20 April 1792 when the French Legislative Assembly declared war on Austria. This launched the War of the First Coalition.
Background
From 1789 to early 1792, the French Revolution gradually radicalised, breaking with old institutions and practices as it went, and targeting defenders of the Ancien Régime. Some of these defenders, or people who were unintentionally caught in the crossfire, emigrated from France to avoid persecution. King Louis XVI himself attempted to escape with his family to Varennes in June 1791, but he was caught. The French king was put under surveillance, and increasingly suspected of conspiring with other European monarchs, who wished to preserve the House of Bourbon in France and restore its pre-revolutionary authority. This was explicitly stated in the Declaration of Pillnitz (17 August 1791) by king Frederick William II of Prussia and emperor Francis II (Austria, Hungary and Bohemia), who called on all monarchs in Europe to 'liberate' Louis. Leading radical revolutionaries called for the complete abolition of the monarchy, but the republican movement was dealt a severe blow in the July 1791 Champs de Mars Massacre. Although this cleared the way for the establishment of the constitutional monarchy in September, it did not secure Louis XVI's position. The uncertain future of the Bourbon monarchy caused tensions to rise between France and other European states.
In early 1792, conservative royalist Armées des Émigrés were forming just across the borders in cities such as Koblenz, readying themselves to invade and end the Revolution with the help of other monarchies. The Girondin majority in the Legislative Assembly favoured war, especially with Austria, in order to display the Revolution's strength and defend its achievements (such as the Declaration of the Rights of the Man and of the Citizen of 1789 and the early beginnings of parliamentary democracy) against a possible return to an (Enlightened) absolutist regime. They cited the Declaration of Pillnitz to justify the urgent need to strike first. Many of the French revolutionaries wanted to spread their Revolution to other countries, and refugees from recently failed revolutions, such as Dutch Patriots and Belgian-Liégois rebels, urged their French comrades to 'liberate' the Low Countries. However, there was a real risk that France would be overwhelmed by foreign forces if a large anti-French coalition were to be formed. This is why many leftist deputies within the Assembly such as Robespierre opposed a war, arguing France was not ready for it and could lose all progress (as they saw it) made thus far during the Revolution.
Preparations
Diplomacy
Major-general Charles François Dumouriez was appointed Minister of Foreign Affairs in March 1792, and by mid-April had managed to obtain the neutrality of all European great powers except Austria and Prussia through diplomacy. Meanwhile, he organised plans to incite a rebellion in the Austrian Netherlands by cooperating with the Committee of United Belgians and Liégeois, who represented remnants of the rebel armies formed during the recently failed anti-Austrian Brabant Revolution and Liège Revolution (August 1789 – January 1791).
Strategy
Finally, France declared war on Austria on 20 April 1792. Dumouriez planned to defeat the Austrian army within 15 days to achieve a successful quick victory. From Dunkirk to Strasbourg, the French northern frontier comprised 164,000 soldiers, divided into three armies under the leadership of general Lafayette (Armée du Centre; targets: from Givet to Namur and Liège), marshal Luckner (Armée du Rhin; targets: Flemish cities such as Menen and Kortrijk) and marshal Rochambeau (Armée du Nord; targets: Quiévrain, Mons and Brussels).
The invasion heavily relied on the presumption that Belgian and Liégois patriotic rebellions would break out spontaneously the moment French troops crossed the border, aiding them in driving out the Habsburg forces as they had done themselves 2.5 years before. Dumouriez assured his fellow ministers:
As soon as the French army enters the Belgian provinces, it will be helped by the people, who are ashamed of their own futile revolutionary efforts of [1789–1790]. They will join forces with our troops and will easily drive the dispersed hordes of Austrian mercenaries from their towns or scatter them. Paris will be defended on the banks of the Meuse. For the Country of Liège, the one most worthy of freedom of all those who have raised its flag, our negotiators will depart to dictate a wise peace, which we will under no circumstances spoil by the spirit of conquest.