The following account of the battle is from the Duc de Sully's memoirs (as translated in Julien Coudy's TheHuguenot Wars):
The end of the footpath to Arques is dominated by a long curved hillside, covered by a thick wood. Below is an area of cultivated land through the middle of which passes the main road leading to Arques, having thick hedgerows on each side. Still farther down, on the left hand, below the cultivated field, is a sort of large marsh, or swampy land. A village named Martinglise marks the hillside about half a league from the causeway. The Duke of Mayenne's entire army was camped in and around this village. The king knew well that he could be accused of rashness in trying to resist an army of 30,000 men, having himself hardly more than 3,000. But, apart from the fact that he could scarcely hope for a place more favorable to his limited force, and realizing the danger of retreating, he felt that the weakness of his own party obliged him, at the very beginning, to deliver an outstanding blow. He neglected nothing that could in any way compensate for his limited numbers. He had deep ditches dug above and below the footpath, as well as on the lower side of the main road. He posted 1,200 Swiss on the two sides of this road. He posted 600 foot soldiers to defend the upper trenches, and between 1,000 and 1,200 others in a chapel which stands halfway between the upper and lower trenches. This was his entire infantry. He divided his cavalry, a total of only 600 men, in two equal groups. He took one half with which he stationed himself between the wood and the road, and had the others go down, in separate platoons, between the road and the swampland, so as to more or less fill the space between. He never went to bed that night, fearing that the enemy might gain control of the footpath; he guarded it himself. In the morning he had some food brought to him in a ditch, and called his principal officers to breakfast there with him. He had thought that after that he might perhaps have a few minutes of rest, but the guards came to inform him that the army of the League was marching toward him in battle array.
At this news he ordered the Viscount de Chartres, Palcheux, Brasseuse, Avantigny, and three or four others to advance into the wood to take a few prisoners. They came back very shortly leading captive the Count de Belin. Smiling, the king went forward to meet him and put his arms around him. The count, who was glancing around in all directions looking for an army and seeing almost no one, answered him by expressing his surprise at finding so few soldiers around the king. "You don't see them all," the king said with his usual gaiety, "for you aren't counting God and Justice who are helping me." Accustomed as I was to seeing this prince, I could but admire the calm serenity of his countenance, for, on an occasion all the more desperate because he had the time to realize his situation, his face had an air of both confidence and alertness, so that it seemed to the soldiers he was something more than human and they were inspired in their turn to match the bravery of their leader.
The Duke de Mayenne first launched an attack on the upper trenches with a a squadron of his foot soldiers who seemed to be refusing to fight because they had at their head only foot soldiers like themselves; they even pretended to surrender, and our men were so deceived by this feint that they let them advance as far as the trench, from which they chased our own men, and from this point of vantage hampered us tremendously. I soon lost sight of what was going on in the direction of the wood, because the marsh side, where I was posted with ten of my men, was at that moment attacked by a squadron of between 800 and 900 horse. At the approach of this much superior troop, we gathered together about 150 horsemen, and drove them back as far as the bend of the little valley where, having run into four other squadrons, we were obliged to retrace our steps, until, finding in our turn the Count d'Auvergne, who led to our aid the other 150 horse, we fought back the enemy cavalry for a second time. But this game could not go on forever. Soon 300 more cavalry came to join the enemy troop, which obliged us to retreat, and we returend to the chapel in disorder. There by good fortune our infantry, who occupied it, stopped their cavalry short, and engaged in a struggle in which Sagonne and several other officers were killed.
The Duke de Mayenne having ordered up all the rest of his foot soldiers to attack the chapel, we finally gave up this post. Overwhelmed by numbers, we also abandoned the sunken parts of the road, and indeed the road itself. It was the beginning of a rout. The result would have been disastrous, had we not encountered the battalion of Swiss, who bore the shock of the attack and permitted us to continue the fight. As far as I was concerned, nothing could have been more opportune. My horse having been wounded and fallen dead at this moment, I mounted another. So as to overcome the brave resistance of our Swiss, the enemy chose to take 500 horse along the path bordering the marsh. This would have taken us from behind, and surrounded us easily along with the Swiss and the rest of our troops; but by great good luck the horses, having got too close to the swamp, became bogged in the mire, and the riders got out with a great deal of difficulty, leaving their arms behind.
The fighting had been going on in this way for some time -- that is to say, our forces were holding out-- when we began to succumb to exhaustion. On our side the same individuals were always in action, whereas our enemies were reinforced and increased constantly. A large part of our brigade was disarmed and unhorsed. In this extremity, I was deputized by the troop to inform the king of our situation and to ask for reinforcement. I met this prince who was passing in our sector. "My friend," he said, "I have no one to send you, but you must not lose courage." Actually, his own sitatuion was no better than ours. Nevertheless, he turned to Monsieur le Grand, and told him to follow me with everyone he could muster from above the roadway. I returned to my troop and told them with feigned delight of help on which I really counted but little. Everyone came to life again, and at this moment here were unbelievable feats of valor; covered by a dense fog, which concealed the enemy from us, we were aware of only a small part of our real danger. When this fog thinned away, the rays of the sun revealed us to the enemy, and we saw their entire army advancing against us. It was already so close that no one of us flattered himself that he could get so far as the end of the footpath, which would have been our last rallying point, and we thougt only of selling our lives as dearly as possible.
Our salvation came from what we had considered our greatest misfortune. The thick fog had made it impossible to use the cannons of the chateau of Arques, but as soon as the enemy was visible they were fired with such accuracy and with such terrible effect that, although we had only four pieces, the enemy was thrown into confusion. Four other salvos followed very rapidly; the entire enemy army, which was completely within range, could not support the fire and retreated in disorder onto the flank of the hill. A few minutes later, all this frightening multitude disappeared, astonished, no doubt, by its losses and replused by a resistanc which the Duke de Mayenne had in no way foreseen.
The king, after a battle which covered him with glory, retired to Arques; from there he went to Dieppe, constantly pursued by enemy groups and engaging in continual skirmishes which I shall not describe, since none of them were of such interest as the one at Arques. Even so, in the course of one of these encournters the king found himself in the midst of an even more obvious personal danger; thinking himself far from any enemy, and in a field practicing with us a sort of miltary game, he was fired on by 200 fusiliers who lay in ambush, flat on the ground between two hedgerows and not more than 200 paces from the place where we stood.
It is certain that anyone but Henry would have been completely crushed before receiving the aid that was on its way to him; but by his bravery and his ability to dispute every inch of gorund he gained time for 4,000 English and Sotch sent to him by Queen Elizabeth across the Channel, and this reinforcement was soon followed by an even greater one led to him by the Count de Soissons, Henry of Orleans, the Duke de Longueville, D'Aumont, and De Biron. He would have been in less dnager at Dieppe, had it not been for the Count de Soissons, who amused himself by bickering over questions of command instead of hastening to the help of the king.
Mayenne dared not wait for all these troops to join forces; he vanished with his army, leaving the king master of the countryside.