Beginning April 27, 1859, Du Pin took part in the Italian campaign as chief of staff of the cavalry division of the 1st Corps of the Army of Italy, fighting in the Battle of Magenta.

He was appointed on November 17, 1859, head of the topographical service of the military expeditionary corps in China, embarking the following December 5 to the East; he took part in the Battle of Palikao and was cited by General de Montauban for the taking of the forts of Peiho on August 21, 1860.

On October 6, 1860, he scaled the walls of the Old Summer Palace in Beijing with a few men, allowing the Franco-British to take possession of the structure without a fight.

In January 1861, he obtained the authorization to go to Japan for a journey of four months with the journalist Antoine Fauchery; of this stay, he will write a book published in 1868 entitled: Le Japon : mœurs, coutumes, description, géographie, rapport avec les Européens.

[6] This fact raised a considerable amount of accusations against General Charles Cousin-Montauban, Comte de Palikao who had directed the operations of the plundering, causing scandal even to Napoleon III himself.

Marshal Adolphe Niel had warned Randon "of the deplorable effect of such a sale on public opinion", in particular because it seemed improper that a militar, however esteemed, should profit from what to all intents and purposes had been a war plunder executed without respect.

The local insurgency was so vigorous that the Expeditionary Corps struggled to hold the area of Puebla; a private counter-guerrilla unit with international recruits was organized, starting from the port of Veracruz.

To accomplish this mission, on February 20, 1863, Du Pin resumed service at the headquarters in Veracruz and took command of his troops, a group of semi-regular units comprising a hundred men of different nationalities, equipped and paid by the French army.

On April 11, he left the city to face the brigades of Generals Pavon and Carvajal who, with 1200 men, laid siege to Temapache, held by Colonel Llorente and 300 Mexicans allied with the French.

With the pacification of the area south of Tamaulipas, Du Pin was able to focus on the reorganization of his corps of volunteers, bringing it to an effective number of 1000 in addition to 500 regular units.

The superior generals gave Du Pin carte blanche in view of the successes he had achieved, but this meant that the French counter-guerrilla troops took control of several economic circuits and the main trade routes in Tamaulipas, which generated a series of illicit deals, particularly in the trafficking of arms and ammunition.

Obviously, then, the favorable treatment received made Du Pin attract the jealousies of other officers of the French expeditionary corps in Mexico.

The band of soldier-brigands under Du Pin's command, his thick white beard and his striking and bizarre Mexican-Hungarian uniform with his pistols in his belt, became a distinctive feature of the French campaign in Mexico in those years.

Serving the cause of Emperor Maximilian I of Mexico, he soon decided to close the military operations in the area and in April 1865 he returned to France, being replaced by Captain Ney d'Elchingen.

Even though it is undeniable that Du Pin made his fortune in Mexico, he was able to bring indisputable results to France from a military point of view, even if at the same time in his homeland a press campaign began to attack him and his work, in particular for the barbaric ways of his administration.

Appreciating the commitment of the soldier, the sovereign placed him at the disposal of Marshal Bazaine without, of course, consulting the latter, who was obviously disappointed to have Du Pin among his men because there was bad blood between the two.

[14]He was a sort of condottiero of the 16th century, a white-haired captain of fortune, proudly leading his "lost boys" [...] I can still see him before me, wearing the striking and bizarre uniform he had adopted: a red dolman open at the front, with a flannel shirt adorned with five golden galloons, with Hungarian knots on the sleeves; white breeches down to the knees; and a light gray Mexican sombrero with flat edges, decorated like a bishop's mitre, trimmed with pendants, the whole thing bordered in gold threads.His manuscript entitled Historique de la contre-guérilla des terres chaudes du Mexique (1865) is preserved today at the Service historique de la Défense, with the photographs he took.