Centralized "sister republic" imposed by France on 12 April 1798 after French troops occupied Bern and dissolved the Old Swiss Confederacy; its unpopular unitary constitution provoked civil unrest (the Stecklikrieg of 1802).
Collapsed into Napoleon's Act of Mediation on 19 February 1803, restoring a looser confederal structure of 19 cantons.
Standard English-language survey covering the French invasion, the Helvetic Republic, and the 1803 Act of Mediation in detail; same verified title already used for the predecessor old_swiss_confederacy record, appropriate here too since it treats this exact transition.