Several claims about his early life seem to be untrue: that he graduated from the École Spéciale Militaire de Saint-Cyr, that he fought in the Austro-Italian War of 1859, that he was awarded the Legion d'Honneur though he wore it in several photographs, or that his father was a count.

[3] In July 1862 Duffié was appointed to command the 1st Rhode Island Cavalry Regiment, with the rank of colonel, by that state's governor, William Sprague IV.

Though the 1st Rhode Island's officers initially refused to serve under a foreign-born leader, Duffié soon won them over and reorganized the regiment into a fine fighting unit.

Though he received a promotion to the rank of brigadier general on June 23, 1863, he would not see active duty again until that fall when he was assigned to raise and train cavalry units in West Virginia.

His division of West Virginia volunteers saw action around Lewisburg in December 1863 and as part of General David Hunter's 1864 campaign in the Shenandoah Valley.

He also took part in operations against Confederate guerrilla leader John S. Mosby, promising to capture the so-called "Gray Ghost" and bring him back to Washington.

[10][11][12] After his death, some of Duffié' officers from the 1st Rhode Island Cavalry established a subscription toward a cenotaph to their former leader, which was erected in Providence's[11][3] North Burial Ground in 1889.

The use of the first name Alexandre reveals more of a family and personal habit or rather of Duffié's intention to desert the French army in case of refusal of his request in order to mislead the military authorities.

At the time of his official funeral in Providence, July 10, 1889, nine years after his death, Lieutenant Samuel C. Willis Junior announced his real name: "One correction I wished to make in regard to General Duffié's name.