In 58 B.C. the Helvetii, a Celtic people inhabiting Switzerland, determined to migrate for the shores of the Atlantic and demanded a passage through Roman territory. According to Caesar's statement they numbered 368,000, and it was necessary at all hazards to save the Roman province from the invasion. Caesar had but one legion beyond the Alps. With this he marched to Geneva, destroyed the bridge over the Rhone, fortified the left bank of the river, and forced the Helvetii to follow the right bank. Hastening back to Italy he withdrew his three remaining legions from Aquileia, raised two more, and, crossing the Alps by forced marches, arrived in the neighbourhood of Lyons to find that three-fourths of the Helvetii had already crossed the Saône, marching westward. He destroyed their rearguard, the Tigurini, as it was about to cross, transported his army across the river in twenty-four hours, pursued the Helvetii in a northerly direction, and utterly defeated them at Bibracte (Mont Beuvray). Of the survivors a few were settled amongst the Aedui; the rest were sent back to Switzerland lest it should fall into German hands.
The Gallic chiefs now appealed to Caesar to deliver them from the actual or threatened tyranny of Ariovistus. He at once demanded a conference, which Ariovistus refused, and on hearing that fresh swarms were crossing the Rhine, marched with all haste to Vesontio (Besançon) and thence by way of Belfort into the plain of Alsace, where he gained a decisive victory over the Germans, of whom only a few (including Ariovistus) reached the right bank of the Rhine in safety. These successes roused natural alarm in the minds of the Belgae - a confederacy of tribes in the north-west of Gaul, whose civilization was less advanced than that of the Celtae of the centre - and in the spring of 57 B.C. Caesar determined to anticipate the offensive movement which they were understood to be preparing and marched northwards into the territory of the Remī (about Reims), who alone amongst their neighbours were friendly to Rome. He successfully checked the advance of the enemy at the passage of the Aisne (between Laon and Reims) and their ill-organized force melted away as he advanced.
But the Nervii, and their neighbours further to the north-west, remained to be dealt with, and were crushed only after a desperate struggle on the banks of the Sambre, in which Caesar was forced to expose his person in the mêlée. Finally, the Aduatuci (near Namur) were compelled to submit, and were punished for their subsequent treachery by being sold wholesale into slavery. In the meantime Caesar's lieutenant, P. Crassus, received the submission of the tribes of the north-east, so that by the close of the campaign almost the whole of Gaul - except the Aquitani in the south-west - acknowledged Roman suzerainty.
In 56 B.C., however, the Veneti of Brittany threw off the yoke and detained two of Crassus's officers as hostages. Caesar, who had been hastily summoned from Illyricum, crossed the Loire and invaded Brittany, but found that he could make no headway without destroying the powerful fleet of high, flat-bottomed boats like floating castles possessed by the Veneti. A fleet was hastily constructed in the estuary of the Loire, and placed under the command of Decimus Brutus. The decisive engagement was fought (probably) in the Gulf of Morbihan and the Romans gained the victory by cutting down the enemy's rigging with sickles attached to poles. As a punishment for their treachery, Caesar put to death the senate of the Veneti and sold their people into slavery. Meanwhile Sabinus was victorious on the northern coasts, and Crassus subdued the Aquitani. At the close of the season Caesar raided the territories of the Morini and Menapii in the extreme north-west.
In 55 B.C. certain German tribes, the Usipetes and Tencteri, crossed the lower Rhine, and invaded the modern Flanders. Expeditions to Britain Caesar at once marched to meet them, and, on the pretext that they had violated a truce, seized their leaders who had come to parley with him, and then surprised and practically destroyed their host. His enemies in Rome accused him of treachery, and Cato even proposed that he should be handed over to the Germans. Caesar meanwhile constructed his famous bridge over the Rhine in ten days, and made a demonstration of force on the right bank. In the remaining weeks of the summer he made his first expedition to Britain, and this was followed by a second crossing in 54 B.C. On the first occasion Caesar took with him only two legions, and effected little beyond a landing on the coast of Kent. The second expedition consisted of five legions and 2000 cavalry, and set out from the Portus Itius (Boulogne or Wissant; see T. Rice Holmes, Ancient Britain and the Invasions of Julius Caesar, 1907, later views in Classical Review, May 1909, and H.S. Jones, in Eng. Hist. Rev. xxiv., 1909, p. 115). Caesar now penetrated into Middlesex and crossed the Thames, but the British prince Cassivellaunus with his war-chariots harassed the Roman columns, and Caesar was compelled to return to Gaul after imposing a tribute which was never paid.
The next two years witnessed the final struggle of the Gauls for freedom. Just before the second crossing to Britain, Dumnorix, an Aeduan chief, had been detected in treasonable intrigues, and killed in an attempt to escape from Caesar's camp. At the close of the campaign Caesar distributed his legions over a somewhat wide extent of territory. Two of their camps were treacherously attacked. At Aduatuca (near Aix-la-Chapelle) a newly-raised legion was cut to pieces by the Eburones under Ambiorix, while Quintus Cicero was besieged in the neighbourhood of Namur and only just relieved in time by Caesar, who was obliged to winter in Gaul in order to check the spread of the rebellion. Indutiomarus, indeed, chief of the Treveri (about Trèves), revolted and attacked Labienus, but was defeated and killed. The campaign of 53 B.C. was marked by a second crossing of the Rhine and by the destruction of the Eburones, whose leader Ambiorix, however, escaped. In the autumn Caesar held a conference at Durocortorum (Reims), and Acco, a chief of the Senones, was convicted of treason and flogged to death.