The Helvetian War was described by Julius Caesar in his De Bello Gallico.
The war was prompted by the Helvetii migration from from the their ancestral home (modern area of Switzerland) to Roman Gaul (modern day France. The originator of the idea to migrate was the weathiest Helvetii Orgetorix. For two years the Helvetii planned and prepared for the migration. Emissaries where sent out to the rulers of Gaul seeking alliances.
When the Helvetii finally set out they where met by Ceasar and routed, suffering heavy losses.
See also