As early as 58 BC, Caesar's troops had penetrated what is now southwest Switzerland. In the succeeding centuries a mix of Celtic tribes and Romans rubbed along in peace and prosperity. Aventicum (today Avenches) became the capital, with as many as 20, 000 inhabitants, and numerous other towns (such as Lausanne) flourished.
By the 4th century AD, the Romans had largely pulled out of Switzerland and Germanic tribes stepped into the vacuum. Christianised Burgundians arrived in the southwest in the 5th century and picked up the Vulgar Latin tongue that was the precursor to French. Absorbed by the Franks, in 1032 Vaud became part of the Holy Roman Empire.
In the 12th and 13th centuries the Dukes of Savoy slowly assumed control of Vaud and embarked on the construction of impressive lakeside castles.
The Canton of Bern appreciatively took over those castles when, in 1536, it declared war on Savoy and seized Vaud. Despite the tendency of Bern's bailiffs to siphon off local wealth, by the 18th century Lausanne (the area's capital) was a thriving centre.
The French Revolution in 1789 had heavy consequences for its neighbours. On the urging of Fréderic César de la Harpe, leader of the Liberal Party, the Directorate in Paris placed Vaud under its protection in December 1797. The following month, Vaud was declared independent. In 1803 Napoleon imposed the Act of Mediation that created the Swiss Confederation in which Vaud, with Lausanne as its capital, became one of six separate cantons.