As mentioned a previous post regarding the naval tactic of Crossing the T, the “Otsu” Tactic was a further development in the Imperial Japanese Navy that was meant to address the weaknesses of the T tactic that was found in war games at the Japanese Naval Staff College and in subsequent naval maneuvers. Therefore, if you have not read the post on Crossing the T, I recommend that you do that first to have some context.
The Japanese “otsu” or “L” tactic is a deceptive maneuver meant to create a converging attack; somewhat similar to a pincer movement or double envelopment. During the maneuver, two separate Japanese columns would approach the enemy column. The first formation to engage would attempt to cross the T, while the second would approach from an unexpected flank and attempt to catch the enemy in a crossfire. Thus, the two formations would form the legs of the character otsu (乙) and prevent the enemy from escaping. Prominent Japanese naval thinker Akiyama Saneyuki wrote:
The tactic for opposing a single enemy flag line is the otsu tactic, in which we subject the enemy to a scissoring attack. Once we have succeeded in maneuvering in this formation against him, we can deal him a mortal blow.
After being tested in war games at the Staff College and later during naval maneuvers, the “otsu” tactic was incorporated into the navy’s battle tactics (Evans & Peattie, 1997, p. 77-79).
In the above examples, you can see the first and second divisions maneuvering into position either in front or behind the enemy formation. This illustrates the ultimate goal of the otsu tactic of ideally crossing the T with the enemy while simultaneously catching them in a crossfire to their front or rear.
Subsequent Tactical Planning
Having been made staff officer to the combined fleet in 1903, Akiyama Saneyuki drafted the Combined Fleet battle plan (連合艦隊旋削 rengo kantai sensaku) for the approaching Russo-Japanese War. This plan set forth the responsibilities, formations, and movements of each unit in the fleet. The battle plan called for the First and Second divisions composed of battleships and armored cruisers, to carry out tei (丁) and otsu (乙) tactics. The plan directed that, “the First Division will choose the formation of the enemy that is easiest to attack, forming a line against it, a T, maneuvering so as to put as much pressure as possible on the enemy’s lead ship; it will make efforts to maintain the T formation with respect to the enemy by carrying out simultaneous turns as appropriate.” (Evans & Peattie, 1997, p. 85). It further stipulated:
The Second Division will pay attention to the enemy’s movements with the aim of carrying out a cross-fire or scissoring attack on the enemy formation that is engaging the First Division. It will maneuver as appropriate, either following the First Division or taking an opposite course, so as to form the character otsu with the First Division… Our two divisions will then carry out a fierce attack on the enemy with crossfire.(Evans & Peattie, 1997, p. 85)
Admiral Togo Heihachiro would later note that:
The battle plan described for the Combined Fleet and each of its Divisions was the basis of all engagements in this war. From the Battle of the Yellow Sea to the great Tsushima Battle and other, smaller clashes in between, there was not a single action not based upon it.(Evans & Peattie, 1997, p. 85)
Following the Battle of the Yellow Sea, the T tactic was beginning to come under scrutiny with Togo noting, “I am afraid that the T tactic lets the enemy get away.” Therefore, the battle plans were revised and, after May 10, 1904, made the T tactic a standard of the fleet, but also emphasized the use of the otsu tactic with additional divisions fighting on parallel courses. Specifically:
The First Division, by putting pressure obliquely on the head of the enemy’s second formation, will seek to turn the enemy aside from his course, commence a battle on parallel courses, and subsequently continue the battle in this way. The Second Division will form an “L” with it insofar as circumstances allow and, by fiercely attacking the rear of the enemy force engaged with it, operate cooperatively with it in accordance with the principle of the “L” tactic.(Evans & Peattie, 1997, p. 113)
The Otsu tactic was more or less an evolution of the classic naval tactic of crossing the T. Based on their experiences in the Russo-Japanese War, the IJN worked hard on how to go about maneuvering their ships into advantageous positions during gun battles. Whether or not this tactic is still useful in the modern age is debatable since it was designed in an era when battleships and battle lines were the standard metrics of naval power. Much like crossing the T, it was suited for its time, but technology marches on.
Evans and Peattie (1997) opine that the Imperial Japanese Navy, along with the Royal Navy, was at the forefront of tactical naval theory in the late-19th century. However, by the 1920s, their tactical doctrine became more and more hardened. By the 1930s, it has ossified into dogmatism. Hori Teikichi noted after the war that “this kind of creeping formalism spread until it became a kind of strategic orthodoxy and [the navy] ended up as a smug little society which insisted that all ideas on strategy should conform to this orthodoxy” (p. 510).
If Hori Teikichi’s assessment is valid, then it is unlikely that any amount of tactical genius could have saved the IJN from its extreme conservatism. By the time WWII ended, there had been a distinct shift in naval power away from battleships and battle line tactics in favor of aircraft carriers and power projection. With that, the classic naval tactics developed since the age of sail became more and more irrelevant.
Evans, D.C., & Peattie, M.R. (1997). Kaigun: Strategy, Tactics, and Technology in the Imperial Japanese Navy, 1887-1941. Annapolis, MD: Naval Institute Press.