The French invasion of Switzerland (German: Franzoseneinfall) occurred from January to May 1798 as part of the French Revolutionary Wars. The independent Old Swiss Confederacy collapsed from the invasion and simultaneous internal revolts called the "Helvetic Revolution". The Swiss ancien régime institutions were abolished and replaced by the centralised Helvetic Republic, one of the sister republics of the French First Republic.

Before 1798, the modern canton of Vaud belonged to the canton of Bern, to which it had a subject status. Moreover, the majority of the Francophone Vaudois felt oppressed by German-speaking Bern. Several Vaudois patriots such as Frédéric-César de La Harpe advocated for independence. In 1795, La Harpe called on his compatriots to rise up against the Bernese aristocrats, but his appeal fell to deaf ears, and he had to flee to Revolutionary France, where he resumed his activism.

In late 1797, French general Napoleon Bonaparte, who had just successfully conquered northern Italy and founded the Cisalpine Republic, pressed the French Directory to occupy Switzerland. France's main goal in the invasion was securing access to northern Italy via the Alpine passes, with supplying its war effort and using the military potential of Switzerland as secondary objectives. Due to internal political and social upheaval, the Confederacy could neither reach an arrangement with France nor organize resistance. In September 1797, Switzerland's situation was further aggravated by the removal of François Barthélemy from the Directory, who had defended a position favorable to the Confederacy.

On 10 October 1797, Valtellina, Chiavenna and Bormio, dependencies of the Three Leagues, revolted and with French support seceded from the Confederacy to join the Cisalpine Republic. In December, the southern part of the Prince-Bishopric of Basel was occupied and annexed to France. Soon 10,000 French troops gathered near Geneva. The atmosphere inside Switzerland had changed significantly due to these developments, and many pro-French patriots hoped, and anti-French conservatives feared, that the Revolution would now spread to the rest of the Confederacy, with or without direct French military intervention. France used the dissatisfaction of the rural elites in the dependencies and the Enlightened citizenry in the cantons to stimulate revolutionary excitement.

The first event of what would become known as the Helvetic Revolution happened with a patriot uprising in Liestal in the canton of Basel on 17 January 1798. The rebels demanded equality before the law, erected a liberty tree and burnt down three Vogtei castles by 23 January. On 24 January, the urban elite of Vaud proclaimed the Lemanic Republic (République lémanique) in Lausanne, which became its seat of government. Next, citizens and subjects in countless Swiss cities, cantons and their dependencies rebelled, and after the example of Vaud, more than 40 other short-lived republics were proclaimed in February, March and April throughout the country.

At the invitation of the Lemanic Republic, 12,000 French troops under General Philippe Romain Ménard [fr] invaded Vaud on 28 January. An incident on 25 January, in which two French hussars were killed by Swiss soldiers in Thierrens, was cited as a pretext. They occupied Vaud without resistance and were cheered on by the population, as Bernese troops withdrew to the area of Murten and Fribourg. A second army under General Balthazar Alexis Henri Schauenburg advanced from Mont-Terrible, the former Prince-Bishopric of Basel, towards Bern and demanded its government to put pro-French Revolutionary parties in power. The Bernese refusal to do so was used by the French to justify war. Bernese field marshal Karl Ludwig von Erlach was appointed supreme commander of all Swiss forces, while General Guillaume Brune held the equivalent office over French forces.

Fighting began on 1 March, and the next day there were battles around Lengnau, Grenchen and in the Ruhsel forest between Alfermée and Twann, which ended with the surrender of the canton of Solothurn. On 4 March, Bern's government resigned; nevertheless, its troops continued to resist the French advance. The next day, the Bernese were defeated at Fraubrunnen and the French attained a decisive victory at the Battle of Grauholz, confirming Vaud's secession. Schauenburg then received the capitulation signed the day before by Karl Albrecht von Frisching, leader of the pro-French Reform Party, who was appointed head of a new provisional government. Erlach, who intended to continue resistance after retreating from Grauholz, was murdered near Wichtrach by his own soldiers who mistook him for a traitor. A Bernese victory at Neuenegg on 5 March, which stopped a French advance from the south through Murten and Freiburg, had no effect in the outcome of the war. Bern suffered 700 killed in the fighting, while French losses are unknown.

The surrender of Bern led to even more dependencies across Switzerland declaring themselves independent republics. However, the Directory desired a single central republican state at France's eastern border, not dozens of small ones, and steered towards (re)establishment of national unity, though this time with equality for all its subdivisions. A new Constitution had already been written in Paris by Peter Ochs and approved by the Directory. Many Swiss rebels detested it, and the National Convention in Basel passed a modified version, which was then adopted by many other entities, but the French government insisted on the original. A proposal by Brune on 16 and 19 March to divide Switzerland into three republics (Tellgovie, Hélvetie and Rhodanie) was also overruled.