Duffield was on the staffs successively of Governors Bagley, Croswell, Jerome and Alger, and kept up a lively interest in the
Detroit Light Guard with which he had long been connected. When the call came for volunteers in the Spanish-American War, although the general officers were taken mostly from the Regular Army, it was determined to select some from among the men who had already seen service in the volunteer army, account being taken of their age, condition of health and record in the Civil War. It was in carrying out this purpose that a commission as brigadier general was offered to Duffield and accepted, dating from May 27, 1898. On June 14, he assumed command of a separate Brigade of the
Second Army Corps, composed of the Thirty-third and Thirty-fourth Michigan and Ninth Massachusetts Volunteers. It was the desire of the Government to reinforce General
Shafter's army which had just landed in Cuba, but only one vessel, the transport Yale, was then available, and that could carry only one brigade. There were two brigades in
Camp Alger, and it was determined to take the one which should first report in readiness to move. Duffield's brigade was then on a practice march to the Potomac, but it returned to camp, won in the test and was dispatched to Santiago. In the Battle of July 1, Duffield was assigned to the duty of making a demonstration on the extreme left, at Aguadores, without any means of crossing the stream, and thus coming into the general engagement. The task was performed in a manner of which Shaffer afterwards said, in an interview in Detroit: "As for General Duffield, of your City, he is a soldier, every inch of him. He had a thankless job at the
Battle of Aguadores, but he acquitted himself nobly." A few days afterwards at Siboney, Major General
Young was taken ill, and the command of his division was turned over to Duffield, who was in turn attacked with
yellow fever, went into hospital, and later in the month was sent north as a convalescent. He joined his family and spent several weeks with them on the coast of Maine, regaining his health. His last act in connection with the war was as one of the speakers at the Peace Jubilee in Chicago, October 18, 1898. ==Organizations==