During the Spanish–American War, Massachusetts became part of the American fleet blockading the Spanish squadron at Santiago de Cuba.

Hart received a letter of commendation for his command of a steam cutter dispatched to reconnoiter Cabanas Bay for possible landing sites.

Under a withering fusillade of fire from Spanish shore batteries, Hart completed the mission and returned his craft to the ship without any casualties although the skiff was struck several times.

[3][4] The Harts spent their honeymoon at the newly rebuilt luxury resort "The Homestead", in Hot Springs, Bath County, Virginia.

His tenacity was responsible for the U.S. Navy's acquisition of surrendered German U-boats after World War I to learn the details of the technical innovations incorporated in the erstwhile enemy craft.

Hart's plan was to replace the aging S-boats, R-boats, and O-boats to provide area defense of submarine bases.

[8] During this period, Hart successfully advocated the building of large destroyer leaders, later classified as anti-aircraft light cruisers (CLAAs), which became the Atlanta class and others.

He held that position at the commencement of hostilities in World War II between Japan and the United States in December 1941.

[17] Hart continued to hold the position of commander for ABDA naval forces until relieved of operational responsibilities on February 12, 1942.

"[20] Hart was retired with the rank of admiral in July 1942, but recalled to duty in August 1942 as a member of the U.S. Navy's General Board.