Short answers, pulled from the story.
The Gallic army led by Brennus marched on Rome in the year 390 BC. The Senones tribe had entered Italy from the north and plundered territory in Etruria and Latium before Roman forces were defeated at the Battle of the Allia.
Rome invaded the territory of the Insubres in 223 BC and took Clastidium, Acerrae and Mediolanum in 222 BC. After the Second Punic War ended, Rome took Bononia in 196 BC, Placentia in 194 BC, and Mutina in 193 BC.
This victory allowed Rome to annex the Allobrogian territory into a new province called Gallia Transalpina. Later this region became known as Gallia Narbonensis and marked a shift from defending Italian borders to expanding beyond them.
The Cimbri defeated consul Marcus Junius Silanus in 109 BC. By 107 BC, the Cimbri and Ambrones together with their Helvetii allies defeated a Roman army near Agendicum and killed Consul Lucius Cassius Longinus during the Battle of Burdigala.
Julius Caesar led a series of campaigns through Gaul from 58 to 50 BC that he chronicled in detail. His actions resulted in the near-complete subjugation of the country between the Atlantic Ocean and the Rhine River and brought most of western Europe under direct Roman control.
Marcus Vipsanius Agrippa marched against the Aquitani between 40 and 37 BC and successfully defeated them. Later, from 28 to 27 BC, Marcus Valerius Messalla Corvinus suppressed another revolt in Aquitania and celebrated a triumph in Rome for this achievement.