Though Caesar had reduced Gaul, he perceived clearly that a great deal was still wanting to make his conquest secure and lasting.  That extensive country, inhabited by a multitude of populous and fierce nations, had been rather overrun than conquered.  The Gauls were not yet broken to the yoke, which they bore with murmuring and discontent.  The ruins of their own strength were still considerable; and they had hopes that the Germans, famous for their invincible courage and their ardent love of liberty, would be at hand powerfully to second any endeavors for the recovery of their freedom; they trusted that the Britons, of their own blood, allied in manners and religion, and whose help they had lately experienced, would not then be wanting to the same cause.  Caesar was not ignorant of these dispositions.  He therefore judged, that, if he could confine the attention of the Germans and Britons to their own defence, so that the Gauls, on which side soever they turned, should meet nothing but the Roman arms, they must soon be deprived of all hope, and compelled to seek their safety in an entire submission.

These were the public reasons which made the invasion of Britain and Germany an undertaking, at that particular time, not unworthy a wise and able general.  But these enterprises, though reasonable in themselves, were only subservient to purposes of more importance, and which he had more at heart.  Whatever measures he thought proper to pursue on the side of Germany, or on that of Britain, it was towards Rome that he always looked, and to the furtherance of his interest there that all his motions were really directed.  That republic had receded from many of those maxims by which her freedom had been hitherto preserved under the weight of so vast an empire.  Rome now contained many citizens of immense wealth, eloquence, and ability.  Particular men were more considered than the republic; and the fortune and genius of the Roman people, which formerly had been thought equal to everything, came now to be less relied upon than the abilities of a few popular men.  The war with the Gauls, as the old and most dangerous enemy of Rome, was of the last importance; and Caesar had the address to obtain the conduct of it for a term of years, contrary to one of the most established principles of their government.  But this war was finished before that term was expired, and before the designs which he entertained against the liberty of his country were fully ripened.  It was therefore necessary to find some pretext for keeping his army on foot; it was necessary to employ them in some enterprise that might at once raise his character, keep his interest alive at Rome, endear him to his troops, and by that means weaken the ties which held them to their country.

From this motive, colored by reasons plausible and fit to be avowed, he resolved in one and the same year, and even when that was almost expired, upon two expeditions, the objects of which lay at a great distance from each other, and were as yet untouched by the Roman arms.  And first he resolved to pass the Rhine, and penetrate into Germany.