Julian had been sent to Gaul immediately after he had received the purple at Mediolanum, with a feeble retinue of 360 soldiers. At Vienna, where he passed a painful and anxious winter in the hands of those ministers to whom Constantius had intrusted the direction of his conduct, the cæsar was informed of the siege and deliverance of Augustodunum. That large and ancient city, protected only by a ruined wall and pusillanimous garrison, was saved by the generous resolution of a few veterans, who resumed their arms for the defence of their country. In his march from Augustodunum, through the heart of the Gallic provinces, Julian embraced with ardour the earliest opportunity of signalising his courage. At the head of a small body of archers and heavy cavalry, he preferred the shorter but the more dangerous of two roads; and sometimes eluding, and sometimes resisting, the attacks of the barbarians, he arrived with honour and safety at the Roman camp near Rheims. The aspect of their young prince revived the drooping spirit of the soldiers, and they marched from Rheims in search of the enemy with a confidence which had almost proved fatal to them. The Alamanni, familiarised to the knowledge of the country, secretly collected their scattered forces, and seizing the opportunity of a dark and rainy day, poured with unexpected fury on the rear-guard of the Romans. Before the inevitable disorder could be remedied, two legions were destroyed; and Julian was taught by experience, that caution and vigilance are the most important lessons of the art of war. In a second and more successful action, he recovered and established his military fame; but as the agility of the barbarians saved them from the pursuit, his victory was neither bloody nor decisive. He advanced, however, to the banks of the Rhine, surveyed the ruins of Cologne,[60] convinced himself of the difficulties of the war, and retreated on the approach of winter, discontented with the court, with his army, and with his own success. The power of the enemy was yet unbroken, and the cæsar had no sooner separated his troops, and fixed his own quarters at Sens, in the centre of Gaul, than he was surrounded and besieged by a numerous host of Germans. Reduced in this extremity to the resources of his own mind, he displayed a prudent intrepidity, which compensated for all the deficiencies of the place and garrison; and the barbarians, at the end of thirty days, were obliged to retire with disappointed rage.
The conscious pride of Julian, who was indebted only to his sword for this signal deliverance, was embittered by the reflection that he was abandoned, betrayed, and perhaps devoted to destruction, by those who were bound to assist him by every tie of honour and fidelity. Marcellus, master-general of the cavalry in Gaul, interpreting too strictly the jealous orders of the court, beheld with supine indifference the distress of Julian, and had restrained the troops under his command from marching to the relief of Sens. If the cæsar had dissembled in silence so dangerous an insult, his person and authority would have been exposed to the contempt of the world; and if an action so criminal had been suffered to pass with impunity, the emperor would have confirmed the suspicions which received a very specious colour from his past conduct towards the princes of the Flavian family. Marcellus was recalled and gently dismissed from his office. In his room Severus was appointed general of the cavalry; an experienced soldier, of approved courage and fidelity, who could advise with respect and execute with zeal and who submitted, without reluctance, to the supreme command which Julian, by the interest of his patroness Eusebia, at length obtained over the armies of Gaul.