Use of the term "commodore" in the Royal Navy dates to the mid-17th century. Dutch in origin, it was first used in the time of William III of Orange. There was a need for officers to command squadrons, but it was not deemed desirable to create new admirals. Captains assigned squadron command were given this title, but it was not an actual rank. The officer so designated kept his place on the list of captains. In 1748 it was established that captains serving as commodores were equal to brigadier generals.
The Royal Navy commodore eventually became split into two classes. Those of the first class had a captain under them to command their ship. Those of the second class commanded their own ship as well as the squadron. In 1783, commodores of the first class were allowed to wear the uniform of a rear admiral, a distinction which continued until the two classes of commodore were consolidated in 1958.
Commodores of the United States Navy have had a more complicated history. Congress was unwilling to authorize any admirals in its service until 1862, so considerable importance was attached to the office of commodore. Like its Royal Navy counterpart, the American commodore was not a higher rank, but a temporary assignment for captains. As Herman Melville wrote in White Jacket, 1849,
The practice was not reserved to captains in the earlier days. Captain Isaac Hull, chafing at not being able to progress further in rank, wrote in 1814 that, if no admirals were to be authorized, something should be done to prevent "every midshipman that has command of a gunboat on a separate station taking upon himself the name of Commodore."
Eventually the title of commodore was defined more strictly, and was reserved for captains so designated by the Navy Department, although the practice of retaining the title for life added some confusion. In 1857, Congress established the grade of Flag Officer. This generic title was intended "to promote the efficiency of the Navy," but differed little from the previous practice. Like the courtesy-title commodores, "flag officers" reverted to captain once their squadron command assignment was completed.
Because of the acute need for officers at the beginning of the American Civil War, naval tradition was ignored and commodore became for the first time a permanent commissioned rank. Eighteen were authorized on July 16, 1862. The rank title also lost its "line command" status when, in 1863, the chiefs of the Bureaus of Medicine and Surgery, Provisions and Clothing, Steam Engineering, and Construction and Repair were given the rank of commodore.
The rank of commodore continued in the US Navy until 1899, when the Naval Personnel Act made all commodores into rear admirals. The reason, according to Laws Relating to the Navy, 1919, was "... on account of international relationships, the consideration of which caused the Navy Department to regard the complications confronting it as inimical to the honor and dignity of this nation, because of the adverse effect upon its high ranking representatives in their association with foreign officers." US commodores were not being treated as flag-level officers by other navies, or given the respect the Navy Department thought was their due.
As it would have been expensive to increase the pay of all the former commodores to the level of rear admirals, Congress specified that the lower half of the rear admirals? list have pay equal to brigadier generals of the army. If there were an odd number of rear admirals, the lower half of the list was to be the larger. All rear admirals, upper or lower half, were equal to major generals, flew a flag instead of a broad pennant, and were entitled to a thirteen gun salute. The Supreme Court held that the rank of commodore had been removed from the navy, leaving it without a rank equivalent to brigadier general. This act disgruntled brigadier generals, who could now be outranked by officers who were their juniors in terms of service. This was a point of inter-service controversy, and in 1916 the army made its brigadier generals equivalent to rear admirals (lower half). Thus, rear admirals (lower half) were equal to major generals, and brigadier generals were equal to rear admirals (lower half), but major generals still out-ranked brigadier generals.
During the naval expansion during World War II, the Navy Department was concerned that the appointment of more flag officers would create a glut of admirals after the war. However, some captains were holding commands of higher responsibility, and needed to be recognized. Admiral Ernest King[?] proposed bringing back the old rank of commodore for these officers. President Roosevelt agreed, though he specified that this rank be restricted to line officers. The Navy's one-star officer reappeared in April 1943. In practice, staff corps officers could also become commodores. By the end of the war, there were over one hundred commodores in service. Very few of the wartime commodores were promoted to rear admiral. Promotions to commodore ended in 1947, and all had left the navy by 1950.
The one-star rank appeared again in 1982 with the title of "Commodore Admiral." The rank became simply "Commodore" the next year, and went back to rear admiral (lower half) in 1985. At that time it was mandated that rear admirals (lower half) would wear only one star, as the two stars authorized previously caused some resentment among US Army and US Air Force brigadier generals.