Series of US naval exercises in the interwar period
The Fleet Problems were a series of United States Navy exercises conducted in the interwar period, later resurrected by the United States Pacific Fleet around 2016.
The first twenty-one Fleet Problems — labeled by Navy leadership as Fleet Problem I through Fleet Problem XXI — were conducted between 1923 and 1940. The culmination of the Navy's annual training maneuvers, they were unscripted, free play exercises involving large concentrations of ships, airplanes, and troops. Fleet Problem XXII, scheduled for 1941, was canceled because of rising tensions with Japan on the eve of the US's entry into World War II. Following the outbreak of war, Fleet Problems underwent a prolonged hiatus, with other names being used to describe large American naval exercises. However, the term was revived in the 21st century under Admiral Scott H. Swift, with Fleet Problem XXIII through XXVIII taking place in the Pacific from 2016 on. Since 2018, the US Navy has not publicly used the historical roman numeral numbering scheme. 
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Interwar Fleet Problems.
The Fleet problems of the 1920's and 1930's were an evolutionary step from existing fleet training exercises. In the 1870's and 1880's, the US Navy was heavily influenced by a reform movement, championed by progressive officers like Admiral William Sims, Admiral Stephen Luce, and Alfred Thayer Mahan. This generation of reform-oriented officers sought to grow the US Navy into a credible battle fleet, which required improving the Navy's equipment, personnel, and training. This movement prompted the creation of the United States Naval Institute in 1873 and the Naval War College in 1884, which both became intellectual hubs for further reform. Modern shipbuilding began in 1882, replacing the largely decrepit wooden-hulled ships built during and after the Civil War with new, steel-hulled vessels, beginning with USS "Dolphin". Finally, after the number of modern warships increased throughout the 1880s, large-scale fleet exercises began in 1889. Large-scale exercises continued on a regular basis until the United States entered World War I in 1917. The US Navy again held a large exercise in 1921, but remained administratively divided into Pacific and Atlantic Fleets until the creation of the United States Fleet in 1922. This large concentration of peacetime forces under a unified staff and commander (a position known as the Commander-in-Chief, US Fleet, or CINCUS), Admiral Hilary P. Jones, set the stage for the mass exercises that would become known as fleet problems.
Fleet Problem I.
From their first announcement, the fleet problems were national news. On 25 December 1922, the New York Times reported about the upcoming exercises for the first time, proclaiming that "all eighteen of the battleships which the United States Government is permitted to retain by the five-power naval treaty will be engaged in these manoeuvres, representing an aggregate of 500,650 tons." 
Fleet Problem I was held off the coast of Panama in February and March 1923. 165 ships and nearly 40,000 men, sailing from both coasts of the United States, participated in the exercise. The United States Battle Fleet, which constituted the attacking Black force, was tasked with attacking the Panama Canal. Shortly after the ships of the United States Scouting Fleet—which, playing the Blue Force, was tasked to defend the Panama Canal—the canal into the Pacific, Black Force launched a simulated air raid. Two battleships, USS "New York" (BB-34) and USS "Oklahoma" (BB-37) simulated aircraft carriers modeled after the under-construction USS "Langley" (CV-1). A single plane launched from "Oklahoma"—representing a 15-plane squadron—dropped 10 miniature bombs and theoretically "destroyed" the spillway of the Gatun Dam. This trapped the ships of the defending Blue Force in the Gulf of Panama, and allowed the Black Force to make a unopposed surface attack on the coast, ending the fleet problem. The exercise ended with a gunnery exercise, with the fleet sinking the pre-dreadnought battleship USS "Iowa""." Fleet Problem I was widely regarded as a success, including by members of Congress and reporters who had observed the fleets in action, setting the stage for a repeated experimentation in future years.
Fleet Problems II, III, and IV.
Fleet Problems II, III, and IV all represented different phases of the same scenario, exploring the initial moves in a notional war against Japan, and took place within the same two months of 1924. These exercises allowed the Army and Navy to validate and practice portions of War Plan Orange, the operations plan for a hypothetical war against Japan in the Pacific, which was formally adopted by the Joint Army and Navy Board later that year.
Fleet Problem II.
Fleet Problem II, which ran from 2 to 15 January, followed the movement of the Battle Fleet, designated as the "Blue Force", from its base on the West Coast to Panama. This was designed to simulate the first leg of an advance from Hawaii towards Asia, especially how well the ships could handle the long transoceanic voyage. During Fleet Problem II, the Navy refined at sea refueling techniques, including refueling side-by-side for the first time between the oiler "Cuyama" (AO-3) and three other ships.
Fleet Problem III and Grand Joint Army-Navy Exercise No. 2.
During Fleet Problem III, the Scouting Force, designated the "Black Force," transited from its homeport in the Chesapeake Bay towards the Panama Canal from the Caribbean side. Once in the Caribbean, the naval forces involved in Fleet Problem III joined with the 15th Naval District and the Army's Panama Division in a larger joint exercise. The Blue force defended the canal from an attack from the Caribbean by the Black force, operating from an advance base in the Azores. This portion of the exercise also aimed to practice amphibious landing techniques and transiting a fleet rapidly through the Panama Canal from the Pacific side.
Black Fleet's intelligence officers simulated a number of sabotage operations during the course of Fleet Problem III. On 14 January, Lieutenant Hamilton Bryan, Scouting Force's Intelligence Officer, personally landed in Panama with a small boat. Posing as a journalist, he entered the Panama Canal Zone. There, he "detonated" a series of simulated bombs in the Gatun Locks, control station, and fuel depot, along with simulating sabotaging power lines and communications cables throughout the 16th and 17th, before escaping to his fleet on a sailboat.
On the 15th, one of Bryan's junior officers, Ensign Thomas Hederman, also snuck ashore to the Miraflores Locks. He learned the Blue Fleet's schedule of passage through the Canal from locals, and prepared to board USS "California" (BB-44), but turned back when he spotted classmates from the United States Naval Academy - who would have recognized and questioned him - on deck. Instead, he boarded USS "New York" (BB-34), the next ship in line, disguised as an enlisted sailor. After hiding overnight, he emerged early on the morning of the 17th, bluffed his way into the magazine of the No. 3 turret, and simulated blowing up a suicide bomb - just as the battleship was passing through the Culebra Cut, the narrowest portion of the Panama Canal. This "sank" "New York", and blocked the Canal, leading the exercise arbiters to rule a defeat of the Blue Force and end that year's Grand Joint Army-Navy Exercise. Fleet Problem III was also the first which USS "Langley" (CV-1) took part in, replacing some of the simulated aircraft carriers used in Fleet Problem I.
Fleet Problem IV.
The 1924 series culminated with Fleet Problem IV, running from 23 January to 1 February. Designed to simulate offensive amphibious operations against Japan, the Blue Fleet was based in Panama—simulating US forces based in the Philippines—while the Black Fleet, made up of the Special Service Squadron, was tasked with defending Puerto Rico—simulating Japanese defenders of Okinawa. Over the week of gameplay, Black aircraft attacked Blue forces consistently, but failed to prevent an amphibious landing - which allowed the US Marine Corps to test their new landing craft, the Christie amphibious tank, and combat logistics techniques. Fleet Problem IV met with serious criticism for having high levels of notional units - units that existed on paper, not in real life. Nearly 10% of Blue's ships and almost 70% of Black's forces were simulated. Vice Admiral Newton McCully argued that "In all exercises, constructive forces or features should be reduced to a minimum," and no later Fleet Problem used the same high level of simulated forces.
Fleet Problem V.
Fleet Problem V ran from 23 February to 12 March 1925. The Black Force, the aggressor, was formed from the United States Battle Fleet, including the US' first aircraft carrier, USS "Langley" (CV-1), along with two seaplane tenders. This gave Black 80 aircraft, while Blue Force, formed out of the Scouting Fleet, was allotted only about 30 floatplanes, half of which were fictional. Even these had limited utility, since the floatplanes carried by , one of Blue Force's battleships, could not be launched for lack of a working catapult. The two fleets made only limited contact on 10 March, when several Blue submarines spotted and attacked the Black fleet, before being sunk by Blue escorts. By the conclusion of the exercise on 15 March, the two fleets had yet to make contact. Admiral Robert Coontz, then serving as CINCUS, remarked that "in some respects, it was a most interesting problem, as it showed that fleets might pass each other unawares." To avoid this issue in later exercises, Fleet Problems VI and VII both began with the two fleets in close proximity to one another. 
While lacking a climactic battle, the exercise did improve the Navy's adoption of emerging technologies. "Langley"'s positive performance helped speed the completion of aircraft carriers and , and increased the role of aircraft onboard battleships and cruisers. The observed limitations of existing S-class submarines spurred a requirement for larger "fleet submarines," with the range and speed to act as scouts for the battle fleet. Both fleets failed to observe radio silence, allowing their opponents to collect transmissions, perform cryptanalysis, and in Black's case, to break Blue's codes. Throughout the exercise, both sides were able to locate the other through these methods.
While returning to San Francisco later that month, the fleet was followed closely by the Japanese tanker "Hyatoma Maru", leading the fleet to tighten its communication security in an attempt to prevent espionage. Fleet Problem V was immediately followed by Grand Joint Army-Navy Exercise No. 3, in which the US Fleet simulated an attack on the Hawaiian Islands to assist the Army in testing and improving their defenses of the territory. The exercise was a failure for the Army; the fleet was able to transit from California to Hawaii undetected by Army patrol planes, and landed the 1st Marine Provisional Marines on Molokai. A series of other contested amphibious landings followed, and by the time the exercise ended on 27 April 1925, almost 30,000 Marines (most fictional, represented by smaller units) were advancing on Scofield Barracks, well into the interior of Oahu. Five days of extensive debrief followed the exercise, which CINCUS Admiral Coontz described as "the best I've ever seen." Army and Navy leadership widely agreed on the need to improve Hawaii's defenses, increasing the size of the garrison and expanding aircraft bases. They disagreed, however, on the efficacy of land-based aircraft against a battle fleet. Army pilots thought highly of their ability to find and defeat ships, reinforcing a misconception that continued until late in World War II, when the Army's failure to effectively sink Japanese ships with land-based aircraft became evident.
Following the conclusion of this exercise in late April, the US Scouting Fleet returned to the East Coast via the Panama Canal, while much of the US Battle Fleet continued on to Australia and New Zealand. The Battle Fleet's voyage lasted until October of 1925, and was the only time that the majority of the interwar US Navy left the Western Hemisphere.
Fleet Problem VI.
Fleet Problem VI, held in February 1926, was the shortest in the series. The entire event lasted only 64 hours and took place exclusively in a small maneuver area off the coast of Panama. It was designed to test a discrete portion of War Plan Orange: the ability of an American naval force to relieve an Army garrison under siege in Manila. Blue (playing the US) was tasked with protecting convoys en route to the notional base, while Black was tasked with attacking the convoys. It also featured further tests of