L'Enfant served first as a volunteer and at his own expense. "In February, 1778," we read in an unpublished letter of his to Washington, "I was honored with a commission of captain of engineers, and by leave of Congress attached to the Inspector-general.... Seeing [after the winter of 1778-9] no appearance of an active campaign to the northward, my whole ambition was to attend the Southern army, where it was likely the seat of war would be transferred." He was, accordingly, sent to Charleston, and obtained "leave to join the light infantry, under Lieutenant-Colonel Laurens; his friendship furnished me," he relates, "with many opportunities of seeing the enemy to advantage."[80]

Not "to advantage," however, did he fight at Savannah, when the French and Americans, under d'Estaing and Lincoln, were repulsed with terrible loss. The young captain was leading one of the vanguard columns in the American contingent and, like d'Estaing himself, was grievously wounded. He managed to escape to Charleston. "I was," he said, "in my bed till January, 1780. My weak state of health did not permit me to work at the fortifications of Charleston, and when the enemy debarked, I was still obliged to use a crutch."[81] He took part, however, in the fight, replacing a wounded major, and was made a prisoner at the capitulation. Rochambeau negotiated his exchange in January, 1782, for Captain von Heyden, a Hessian officer.

"Your zeal and active services," Washington wrote back to L'Enfant, "are such as reflect the highest honor on yourself and are extremely pleasing to me, and I have no doubt they will have their due weight with Congress in any future promotion in your corps."[82] They had, in fact, in the following year, when, by a vote of the assembly, L'Enfant was promoted a major of engineers, 1783.

His knowledge of the art of fortification, his merit as a disciplinarian, the part he had taken, as he recalls in a letter to Count de La Luzerne,[83] in devising the earliest "system of discipline and exercises which was finally adopted in the American army" (all that was done in that line was not by Steuben alone), rendered his services quite useful. His gifts as an artist, his cleverness at catching likenesses made him welcome among his brother officers. He would in the dreary days of Valley Forge draw pencil portraits of them, one, we know, of Washington, at the request of Lafayette, who wanted also to have a painted portrait. "I misunderstood you," the general wrote him from Fredericksburg, on September 25, 1778; "else I would have had the picture made by Peale when he was at Valley Forge. When you requested me to sit to Monsieur Lanfang"—thus spelled, showing how pronounced by Washington—"I thought it was only to obtain the outlines and a few shades of my features, to have some prints struck from."

Some such pencil portraits by L'Enfant subsist, for example in the Glover family at Washington, and are creditable and obviously true-to-nature sketches.

Whenever, during the war or after, something in any way connected with art was wanted, L'Enfant was, as a matter of course, appealed to, whether the question was of a portrait, of a banqueting hall, of a marble palace, a jewel, a solemn procession, a fortress to be raised, or a city to be planned. A man of many accomplishments, with an overflow of ideas and few competitors, he was the factotum of the new nation. When the French minister, La Luzerne, desired to arrange a grand banquet in honor of the birth of the Dauphin (the first one, who lived only eight years), he had a hall built on purpose, in Philadelphia, and L'Enfant was the designer. Baron de Closen, Rochambeau's aide, writes as to this in his journal: "M. de La Luzerne offered a dinner that day to the legion of Lauzun, which had arrived the same morning (August 2, 1782). The hall which he caused to be built on purpose for the fête he gave on the occasion of the birth of the Dauphin, is very large and as beautiful as it can be. One cannot imagine a building in better taste; simplicity is there united with an air of dignity. It has been erected under the direction of Mr. de L'Enfant, a French officer, in the service of the American corps of engineers." Closen adds that "Mr. Barbé de Marbois,[84] counselor of embassy of our court, is too modest to admit that his advice had something to do with the result."

When peace came, those officers who had fought shoulder to shoulder with the Americans returned home, bringing to the old continent new and fruitful ideas, those especially pertaining to equality and to the unreasonableness of class distinctions. Liberty had been learned from England; equality was from America.

L'Enfant was one of those who went back to France, but he did not stay. He had been away five years and wanted to see his old father, the painter, whose end now was near. A royal brevet of June 13, 1783, had conferred on the officer a small French pension of three hundred livres, "in consideration of the usefulness of his services, and of the wounds received by him during the American war."[85] He sailed for France late in the same year, reaching Havre on the 8th of December.

The Society of the Cincinnati had been founded in May. For the insignia appeal had been made as usual to the artist of the army,[86] L'Enfant, who was, moreover, commissioned by Washington, first president of the association, to avail himself of his journey to order from some good Paris jeweller the eagles to be worn by the members, L'Enfant himself being one. He was also to help in organizing the French branch of the society. Difficulties had first been encountered, for the reason that no foreign order was then allowed in France, but it was recognized that this could scarcely be considered a foreign one. In an unpublished letter to Rochambeau, Marshal de Ségur, minister of war, said: "His Majesty the King asks me to inform you that he allows you to accept this honorable invitation (to be a member). He even wants you to assure General Washington, in his behalf, that he will always see with extreme satisfaction all that may lead to a maintenance and strengthening of the ties formed between France and the United States. The successes and the glory which have been the result and fruit of this union have shown how advantageous it is, and that it should be perpetuated." Concerning the institution itself the minister wrote: "It is equally honorable because of the spirit which has inspired its creation and of the virtues and talents of the celebrated general whom it has chosen as its president."[87]

L'Enfant sent to Washington glowing accounts of the way the idea had been welcomed in France, and told him of the first meetings held, one at the house of Rochambeau, Rue du Cherche-Midi, for officers in the French service, and another at the house of Lafayette, Rue de Bourbon, for French officers who held their commissions from Congress, both groups deciding thereupon to unite, under Admiral d'Estaing as president-general.[88]