When the revolutionary wars broke out, and France was ringed round by a coalition of enemies, the voice of "la patrie en danger" rang in the ears of the young student like a call from the skies. He was twenty-two years of age when two deputies of the Legislative Assembly came down to Caen and made an appeal to the manhood of the country to fly to arms. Decaen, fuming with patriotic indignation, threw down his quill, pitched his calf-bound tomes on to their shelf, and was the first to inscribe his name upon the register of the fourth battalion of the regiment of Calvados, an artillery corps. He was almost immediately despatched to Mayence on the Rhine, where Kleber (who was afterwards to serve with distinction under Bonaparte in Egypt) hard pressed by the Prussians, withdrew the French troops into the city (March, 1793) and prepared to sustain a siege. Decaen rose rapidly, by reason not merely of his bull-dog courage and stubborn tenacity, but also of his intelligence and integrity. He received his "baptism of fire" in an engagement in April, when Kleber sent a detachment to chase a Prussian outpost from a neighbouring village and to collect whatever forage and provisions might be obtained. He was honest enough to confess--and his own oft-proved bravery enabled him to do so unashamed--that, when he first found the bullets falling about him, he was for a moment afraid. "I believe," he wrote, "that there are few men, however courageous they may be, who do not experience a chill, and even a feeling of fear, when for the first time they hear around them the whistling of shot, and above all when they first see the field strewn with killed and wounded comrades."* (* Memoires 1 13.) But he was a sergeant-major by this time, and remembered that it was his duty to set an example; so, screwing up his courage to the sticking-place by an effort of will, and saying to himself that it was not for a soldier of France to quail before a ball, he deliberately wheeled his horse to the front of a position where a regiment was being shaken by the enemy's artillery fire, and by his very audacity stiffened the wavering troops and saved the situation.