Marines

The Marines (海軍, Kaigun) are the World Government's military sea force. It is present in all the seas around the world, although they maintain only a slight military presence in the four Blues by establishing Branch Offices. The focus of the bulk of its assets on are on the far more dangerous Grand Line, as their main headquarters, Marineford, is located there. They first appeared in Chapter 3 and Episode 1.

Duties
The duty of the Marines is to maintain a law and order throughout the world and impose the will and might of the World Government. They are therefore considered the key strategic development force in the World Government and are expected to obey its orders at will. Sometimes these orders are questionable, however they are expected to carry them out regardless of opinion. Amongst their duties is awarding bounty hunters the bounties on the heads of a criminal. They tolerate bounty hunters as a method of making their job easier but are not unknown to go out bounty hunting themselves. Apprehended criminals who are brought in alive are handed over to be made an example of, the Marines are left to ensuring the exportation of these criminals to prison and/or to their execution.

Marines often work in pairs with one superior officer and a second in command or subordinate who is of slightly low rank. These partnerships may remain even during promotions with both pairs advancing in rank together. In the pair the higher ranked officer will give the orders to the men under their command, while the other acts an adviser or information gatherer while ensuring their partners command is undertaken by the men.

Higher ranked Marines (Admirals and Fleet Admiral) are expected to be on call to led the Marine forces into war. This may take form of comprehending many criminals at once, protecting the World Nobles or calling a Buster Call at tackle a situation. Vice Admirals are the ones who serve on the ships that the Admirals call out and are tasked with carrying out the Admirals orders exactly.

The main purpose of the Marines remain footed in the idea of bringing Justice to any criminal. They use Justice as their constituted policy to manage a world that is made entirely of one ocean much easier. However, the main problem within the Marines is the approach to Justice. They are generally divided between those who support Moral Justice and those who support Absolute Justice with the latter being the code of conduct for the Marines as a rule. Nevertheless, there is little worry from the higher ranked Marines as to how the Justice is carried out so long as it is, leading those who truly support absolute Justice to make unjustified actions in carrying out their duties.

The Marines are also aided by the Shichibukai in terms of maintaining the "Three Great Powers", and as such, are expected to cooperate when necessary. Despite this, several Marines, such as Smoker, Sengoku, and Kizaru thinks of the Shichibukai as no more than pirates. In turn, the Shichibukai has no obligations to obey the Marines unless under the direct orders of the World Government itself.

Rank System
The system of military rank in place for the Marines is fairly straightforward, where each rank is clearly delineated.

Officers and recruits present at Marine Headquarters are considered "Super-elites". All ranks present from "Lieutenant" above are usually trained here, exceptions being those in command of other Marine bases which includes Axe-Hand Morgan and Nezumi.

Rank Insignia
The Marines do not have a system of rank insignia per se. There is significant variation in the kind of uniform worn by officers, with many opting to wear a variety of dress showcasing their own eccentric style. Marines are also allowed to wield whatever weapon they are most comfortable with, no matter how unusual it is. They are even free to fight bare-handed. However, as a general rule set down by the author, uniform guidelines are as follows:
 * Upon first entering the Marines, new recruits are given the standard uniform, consisting of a white short-sleeved shirt emblazoned with the Marine emblem, a simplified seagull with "MARINE" underneath it, on the back; a blue neckerchief; dark blue trousers; and a baseball cap with "MARINE" across the front. All officers at the rank of Seaman (一等兵 Ittōhei) and below wear this. The quasi-rank of "Zatsuyō" (雑用 Zatsuyō) adds the kanji 雑 Zatsu ("various" or "odds and ends") to the sleeves, and to the Marine emblem on the back.
 * Once an officer has reached the rank of Petty Officer (伍長 Gochō), he or she can choose from a number of different uniforms, including his or her personal clothes. Only those who really want to appear "sailor-like" will stay with a more traditional uniform.
 * Sailors at the rank of Warrant Officer (准尉 Jun'i) and above are allowed to wear the coat with "Justice" (正義 Seigi) on the back. This coat tends to be worn like a cape, draped over the shoulders with the arms not in the sleeves.
 * For the rank of Lieutenant-Commander (少佐 Shōsa) and above, a suit worn underneath the "Justice" coat is the preferred attire.

There are, of course, exceptions: Jango and Fullbody wear their own clothes with the "MARINE" hat, despite holding the rank of "Seaman Recruit" (三等兵 Santōhei). This may have something to do with the exceptional circumstances by which both of these men were brought into their present situation, or it may simply be lenience on the part of their Captain, Hina.

Commissioned Marine Officers

 * Fleet Admiral (元帥)
 * Head of the Marines
 * Admiral (大将)
 * Governors-General of the Marines; only three such positions
 * Vice Admiral (中将)
 * Rear Admiral (少将)
 * Commodore (准将)
 * Captain (大佐)
 * Commander (中佐)
 * Lieutenant Commander (少佐)
 * Lieutenant (大尉)
 * Lieutenant Junior Grade (中尉)
 * Ensign (少尉)

Infantry and Sailor Division

 * Warrant Officer (准尉)
 * Master Chief Petty Officer (曹長)
 * Chief Petty Officer (軍曹)
 * Petty Officer (伍長)
 * Seaman First Class (一等兵)
 * Seaman Apprentice (二等兵)
 * Seaman Recruit (三等兵) or Apprentice (新兵)
 * Chore Boy (雑用) (not official rank)

History
Exactly how long the Marines have been around for is unknown, so is how they were created.

The oldest known date of their existence was 1472 AOS where Oimo and Kāshī were captured by the Marines and were tricked into guarding Enies Lobby, hoping to free Dorry and Brogy whom they thought had been arrested.

In 1500 AOS, Garp was known as the legendary Marine who cornered Gol D. Roger many times. After the Execution of Roger, the Marines and the Marine Judges went to Water 7 to judge and later execute Tom for supporting him in his pirate deeds by constructing Roger's ship the Oro Jackson.

In 1502 AOS, a platoon of Marines, including five Vice Admirals, were sent to Ohara for the execution of a Buster Call regarding the research of lost history that annihilated the island. Three of the most notable who took part included future admirals Kuzan and Sakazuki as well as Jaguar D. Saul, who sacrificed his life in protection of the island.

After Marshall D. Teach has captured the second division commander of the Whitebeard Pirates, Portgas D. Ace to become a Shichibukai, with Ace's execution being announced, the Marines are preparing to have an all out war against the Whitebeard Pirates, which took place at Marineford. Alongside the Whitebeard Pirates came Monkey D. Luffy and a motley crew of Impel Down breakout convicts, and the war has escalated to its final phase after one-and-a-half hour.

Translation and Dub Issues
Although the literal roman characters "Marine" are displayed prominently throughout the manga on the hats and uniforms of officers and at military installations, the Japanese word that is used is "海軍" (かいぐん) "kaigun", which translates most closely to "Navy".

One of the reasons that Navy is the traditional English translation of the word "kaigun" is the close association that the Imperial Japanese Navy had with the British to the extent that bridge commands in the INS were even issued in English until shortly before WWII. As a result, Viz as well as 4Kids decided to translate it as "Navy". While Viz alters the various "kaigun" kanji to read Navy in English while leaving the word Marine alone on various equipment and personnel, 4Kids, on the other hand, took a different approach. 4Kids opted to erase kanji and to change all instances where the word "Marine" appears in the artwork to read "Navy".

In English, to call the Navy the "Marines" or the "Marine Corps" can be considered a mistake, because Marines (in the English sense of the word) only make up a portion of the Navy's forces. For example, in the American Army the word "Marine" is used to describe an army unit who is deployable on land or at sea, while a strictly seafaring force is called a Navy. Evidence from the series (specifically, the author's frequent use of vocabulary from a variety of mainland European languages) suggests that the use of "Marine" in the Navy's emblem may come from French or German. In both these language "Marine" has the same meaning as the English word "Navy".

Nevertheless, English speaking fans of the series generally accept either term, though some who are fans of the Japanese version (but do not actually speak Japanese) have been known to criticize the use of the word "Navy,". This is normally either as a knee-jerk reaction against the 4Kids dub of the Anime (which uses the term) or out of unawareness that the word "Navy" is a more proper translation of the term "Kaigun". In the end, it is important to remember while Navy or Marines can be used as the correct term for the group, using "Marine" as a direct English translation of the Japanese is incorrect.