On the return of young Saumarez to New York in September 1779, he was strongly recommended by his commanding officer to General Clinton, the commander-in-chief, and, in consequence, was permitted to purchase a company in the Royal Welsh Fusiliers, although he was only 19 years of age. The day following, he embarked with several regiments, under the command of Earl Cornwallis, with the intention of attacking several French West Indian Islands. A few days after leaving New York, a frigate hove in sight, the captain of which gave information to Vice Admiral Arbuthnot, that two days before he had seen a large fleet of men-of-war, under the command of Count De Grasse, very superior in numbers to our convoy, and that he supposed the enemy to be within a day's sail of our ships. Upon this the Admiral made the signal for all the transports to return as expeditiously as possible to New York, where he disembarked.

In December, our young officer embarked with many regiments, under the command of Lieutenant General Clinton, and sailed for South Carolina, to besiege the city of Charlestown, which surrendered on the 12th of May 1780. Soon after this, he was detached with 4000 men, under the orders of Earl Cornwallis, and after marching upwards of one hundred miles took possession of Camden. At this place, our little army became so sickly that we had more than 1100 men in the hospital, which, with many detachments, reduced our number to less than 2000 effective men. The enemy being apprised of this, was induced to collect a force of more than 7000 men, with the intention of attacking and capturing our little army, under the command of General Gates. On the 15th of August, Lord Cornwallis was informed that the Americans were within twelve miles of Camden, and consisted of six to one in numbers more than we had to oppose to them. Upon this, his lordship considered it was too late to think of retreating to Charlestown, and not wishing to abandon our sick in hospital, decided at once that, by attacking the enemy, we had a great deal to gain and little to lose. He accordingly issued an order to march at nine o'clock the same evening. About two hours after, the advance guards of the British and of the Americans encountered each other, as the enemy had begun to march precisely at the same hour: after skirmishing some time, the firing ceased, and both armies waited most impatiently for the dawn of day of the 16th, when they formed, and immediately engaged; the Americans at the same time detaching troops on both the flanks of the British, to prevent their escaping, under the expectation of taking the whole prisoners. On the other hand, the British marched coolly to meet the enemy, although under a very heavy fire of cannon and musketry, until they came within twenty yards of their opponents. Here Lord Cornwallis took off his hat, which was a signal for the line to give three hearty cheers, advance, and, when within a few yards of the enemy, fire a well-directed volley and charge: this was done with such effect that the first line of the Americans ran away and overset their reserve; the result was, that the British killed (mostly with the bayonet), wounded, and took prisoners 300 more than they had men in the field, took seven pieces of brass cannon, 150 waggons, full of all sorts of military stores, camp equipage, &c.

About three weeks after this action, Lord Cornwallis, upon finding the greater number of the sick had left the hospital, issued an order for all the officers' baggage which could possibly be dispensed with to be destroyed immediately, as the little army was going by forced marches in pursuit of the enemy. The troops accordingly marched seventeen successive days, from five o'clock in the evening to eight or nine the following morning, oftentimes with a very scanty allowance, or no provisions, as it was through an exhausted country, without bread, (as the corn mills had been rendered unserviceable,) except some Indian corn used by the cattle, and this corn was taken from the fields. The troops were without tents or any covering to shelter them from the intense heat and heavy rains peculiar to the climate. They had to ford frequently four or five rivers and creeks in a day; some of these were deeper than their waist, and so rapid, that the officers and soldiers found it requisite to tie and support each other. Under these circumstances the men were frequently exposed to a most galling fire from the enemy, strongly posted: if a man was wounded, he was let go down the stream and drowned.

During a march of 1500 miles through South and North Carolina and Virginia, the officers and soldiers were subjected to the greatest sufferings, privations, and hardships, which, (as Lord Cornwallis frequently observed in his despatches,) could not be possibly exceeded, their clothes being worn out, especially their boots and shoes. They were, moreover, almost without wine or spirits, having destroyed the greater part when orders were issued at Camden to lessen the baggage as much as possible, which deprived the officers of the comforts they so much required, and which they had obtained with the greatest trouble and expense: for this sacrifice, they never received the smallest recompence. The officers having the rank of captain were allowed to ride on a march, but in consequence of a requisition made to Lord Cornwallis by Colonel Tarleton, commanding the cavalry, not only for the riding horses, but also for all the cart horses, which were most serviceable to mount his troopers, his lordship most reluctantly compelled every officer to deliver the best of the horses for the cavalry. The captains naturally lent their horses to the officers and men who might require them from illness or otherwise; it was soon found out that they could not be dispensed with, so that cast-off horses were substituted for those they had been obliged to give up.

The little army being nearly exhausted with fatigue, the officers and men became most anxious that, instead of the minor actions and skirmishes to which they were frequently exposed, the enemy would collect all his force and give them an opportunity to fight and end their labours.

On the 14th of March 1781, Lord Cornwallis received intelligence that General Green, with a force five times greater as to numbers than the British, was within ten miles. His lordship determined to attack them the day following, and put his little corps in motion at daybreak of the 15th of March. About noon, he fell in with the enemy most advantageously posted, and formed in three lines; the first, which was behind rails, kept up a most incessant fire, from four six-pounders and musquetry, upon the British troops as they advanced upon a ploughed field, which was very muddy from rain that fell the day before. Notwithstanding all these disadvantages, they marched coolly to the Americans without firing a musket until within a few yards, when they halted to fire a well-directed volley and charged. Upon this, they had to encounter the enemy's second and third lines, which were attacked in the same manner and totally dispersed, leaving their four six-pounders, the only guns they had, in the field, which were bravely taken by the brigade of guards: these four six-pounders were soon after retaken by a charge from Colonel Washington's cavalry, and two of these guns were ultimately taken by Captain Saumarez, who had the command of the left wing of the regiment from the commencement of the action, after Captain Pater, who commanded the royal Welsh Fusiliers, was wounded in the early part of the engagement, and the left wing had been separated from the right wing when in pursuit of the first line of the Americans. The other two six-pounders were also taken by Colonel Tarleton's cavalry: these four guns were all the Americans brought into the field.

The enemy's loss in killed and wounded was very considerable. The Welsh Fusiliers and most of the other corps of Lord Cornwallis's army had about one third of the officers and soldiers in the field killed and wounded, and most unhappily, during the action, the lighted paper of the cartridges set fire to the dried leaves, so that many of the unfortunate wounded, which could not be removed, belonging to the British and Americans, were burnt to death. Earl Cornwallis mentioned in his despatches, "that the conduct and actions of the officers and soldiers that compose this little army, will do more justice to their merit than I can by words. Their persevering intrepidity in action, their invincible patience in the hardships and fatigues of a march of many hundred miles, in which they have forded several large rivers and numberless creeks, many of which would be reckoned large rivers in any other country in the world, without tents or covering against the climate, and often without provisions, will sufficiently manifest their ardent zeal for the honour and interests of their sovereign and their country."

This engagement was generally considered to be the hottest and bloodiest, as well as the best fought throughout the war.[15] The army marched from Guildford the 18th of March for Wilmington, where it arrived the 7th of April. At this place the officers and soldiers fortunately were enabled to supply themselves with a small quantity of wine and spirits, with which they had been without for some months, also with tea and sugar and some clothing, of which they began to be in the greatest need, in consequence of having been compelled to destroy the greatest part of their baggage twice for the good of the service, since they arrived at Camden in June 1780.

On the 25th of April 1781, he marched from Wilmington, North Carolina, for Petersburg, Virginia, a distance of 800 miles: here he arrived on the 20th of May, after undergoing the greatest privations and hardships, which Lord Cornwallis deplored, and felt the distresses of his little army so much that he became very ill with a fever, which prevented the possibility of his lordship's sitting a horse, and made it indispensably requisite for his being conveyed in a waggon over mountains, rivers, and creeks.

On the 4th of July the troops marched from Williamsburg, the capital of Virginia, for James Town, near which place there was a river three miles wide, which the army had to cross. On the 5th, the baggage of the army passed over the river, and some of the troops. The day following, Lord Cornwallis received intelligence that the Marquis De La Fayette with 2000 Americans were within a short distance of the British, with the intention of destroying the rear guard: upon this, his lordship prevented the main body of his little corps from embarking, and placed it in ambush behind a high hill to wait the attack of the enemy. About four o'clock in the afternoon, the Americans began to attack the piquets, which had orders to sustain their ground as long as possible; in consequence, several officers and soldiers were wounded: at length the main body of the enemy formed in front of the British, when the latter, after receiving the repeated discharges of the former from two six-pounders and musquetry, advanced, with the greatest impetuosity, fired a volley and charged, which completely dispersed the Americans, who were pursued until dark, the enemy leaving the two pieces of cannon and more than 300 killed and wounded on the field. Another hour of day-light would in all probability have prevented a single man of the Americans escaping.