The start of the Reformation came just as Bern began to pursue expansionist policies in the west, exploiting the weakness of the House of Savoy, which ruled parts of the Pays de Vaud and the county (but not the city) of Geneva.

The citizens of Geneva were meanwhile locked in a power struggle with their bishop - who was in turn backed by Savoy.

Bern and Geneva thus shared an anti-Savoy interest, and made an alliance of combourgeoisie in 1526.

Matters came to a head in 1536, when Bern acted to prevent what it feared might be a French takeover of Savoy. It invaded Vaud, Chablais and the Pays de Gex (the last two both now in France), and entered Geneva. Fribourg and Valais subsequently joined the expedition, and carved up the area between them, with Bern taking the lion's share.

Geneva was allowed to keep its independence, but Lausanne - which also had a treaty of combourgeoisie - and Vaud as a whole, became subjects of Bern.

After the conquest, Bern immediately imposed the new reformist religion in the area; Lausanne as well as Geneva became centres of French-speaking Protestantism.