Upon returning to the United States, Bernadou was assigned to the recently revitalized Office of Naval Intelligence (ONI). There, he went to work studying world economic conditions with a particular emphasis on the world’s supply of nickel ore. The importance of that commodity resulted from the Navy’s adoption of nickel-processed steel for its new ship construction. He also collected information on foreign seaports, assisting in the writing of a book on international ports and coaling stations. His facility with foreign languages put him in an ideal position to translate articles of value to ONI from French, German, Russian, Swedish, Spanish, and other languages.

Between February 1891 and May 1893, he served in the cruiser Newark. On 1 July 1892, he was promoted to lieutenant (junior grade). He took a leave of absence because of illness from May to July 1893 and then returned to sea in Bennington (Gunboat No. 4). In that warship, he made a cruise to European and Mediterranean waters, there transferring to the protected cruiser Chicago. In September 1894, Bernadou began three years of service at the Torpedo Station located at Newport, R.I. While in that assignment, he was promoted to lieutenant in June 1896. In December of 1897, he went to the Norfolk Navy Yard where he put Winslow (Torpedo Boat No. 5) into commission on the 29th as her first commanding officer.

War with Spain erupted late in April 1898, and Bernadou was soon patrolling the northern coast of Cuba in Winslow. On 11 May, he took Winslow to Cárdenas to take on coal from one of the larger warships there. Upon reporting to the commanding officer of Wilmington (Gunboat No. 8), he received orders to take Winslow into the bay at Cárdenas and scout for mines. Winslow and the revenue cutter Hudson searched the harbor entrance but found no mines. They rejoined Wilmington about noon to make their report. Wilmington’s commanding officer decided to take the three warships into the bay in search of the three Spanish gunboats reported there. Bernadou’s command marked shoal water to Wilmington’s portside during the entry. Upon reaching a point about 3,000 yards from the city, a lookout spied a small, gray steamer moored alongside the wharf. Bernadou then received orders to move his ship in closer to determine whether or not the ship was an enemy warship.

At around 1335, his warship reached a point about 1,500 yards from the object of his interest when a white puff of smoke announced the opening of an artillery duel that would last an hour and 20 minutes. Bernadou responded with Winslow’s 1 pounders, and then enemy batteries ashore joined the deadly contest. Bernadou’s little ship bore the brunt of the Spanish fury, and she soon received a number of direct hits. The first shell to strike Winslow destroyed both her steam and manual steering gear. Bernadou’s crew tried to rig some type of auxiliary steering gear while he steered her with the propellers in an attempt to keep her bow gun unmasked and to present the enemy with as small a target as possible. All at once, however, Winslow swung broadside to the shore batteries. Quickly, a shot knocked out her port main engine. Bernadou then tried to maneuver his warship with the remaining engine to evade Spanish fire and to keep his guns in action. At some point before that time, a shell burst on the top of the forward conning tower; and a fragment from it struck Bernadou in the thigh.

Almost simultaneously, Wilmington and Hudson brought their larger guns to bear on the Spanish shore batteries. The Spanish gunboat received fatal hits, and her crew abandoned her, while the shore batteries slackened fire. Bernadou requested Hudson to tow his all-but-disabled torpedo boat out of action. A towline was passed between the two ships, but it soon parted. Spanish shore batteries continued their fire, and one shell burst near the after engine room hatch killing four of the crew and Ens. Worth Bagley, the first and only American naval officer killed by enemy action in the Spanish-American War.

The towline was finally rerigged, and Winslow, badly damaged, was towed clear of the action. Bernadou relinquished command of the ship to Chief Gunner’s Mate George P. Brady and went over to Wilmington with the rest of the wounded. For his gallantry at Cárdenas, Bernadou received a commendation and advancement (10 numbers) in seniority.

After recovering from his wounds, Bernadou returned to duty at the Bureau of Ordnance where he served from late 1898 until sometime in 1899. No doubt, he resumed the work on perfecting smokeless gunpowder that he had performed previously at Newport, R.I. In 1899, he returned to sea in Indiana (Battleship No. 1). In 1900, he transferred briefly to newly commissioned Kentucky (Battleship No. 6) before joining the training ship Dixie. He made two cruises to Mediterranean waters and served briefly on the South Atlantic Station. On 9 February 1902, probably while still assigned to Dixie, Bernadou was promoted to lieutenant commander. Later that year, he began another tour of duty with ONI in Washington, D.C. That assignment lasted until 1904 when he returned to sea as executive officer of Kearsarge (Battleship No. 5). He served in that billet until sometime early in 1906. After a brief tour of duty at the Naval War College in Newport, R.I., Bernadou went to Europe to serve as naval attaché in Rome and Vienna. On 11 December 1906 while in Europe, Bernadou was promoted to commander. He remained on diplomatic duty until sometime in mid 1908 when complications caused by the wound he received at Cárdenas forced him to return home. Bernadou died at the Naval Hospital, Brooklyn, N.Y., on 2 October 1908. Three days later, he was buried with full military honors in the National Cemetery at Arlington, Virginia.