The borders of Roman Italy, ''Italia'', are better established. Cato's ''Origines'', the first work of history composed in Latin, described Italy as the entire peninsula south of the Alps. According to Cato and several Roman authors, the Alps formed the "walls of Italy". In 264 BC, Roman Italy extended from the Arno and Rubicon rivers of the centre-north to the entire south. The northern area of Cisalpine Gaul was occupied by Rome in the 220s BC and became considered geographically and ''de facto'' part of Italy, but remained politically and ''de jure'' separated. It was legally merged into the administrative unit of Italy in 42 BC by the triumvir Octavian as a ratification of Caesar's unpublished acts (''Acta Caesaris''). The islands of Sardinia, Corsica, Sicily and Malta were added to Italy by Diocletian in 292 AD., coinciding with the whole Italian geographical region. All its inhabitants were considered ''Italic'' and ''Roman''. 