The Marines constitute one of the Three Great Powers in the One Piece world. The aim of the Marines is to ensure that the pirates and other criminals are kept under control and do not wreak havoc throughout the world. With the number of pirates increasing at an incredible rate, the Marines cannot afford to wallow.
When the series started, the Marines had the Shichibukai to encumber the threat of the pirates, but after its dissolution, they are in a pinch. The Marines aren't the only ones who fight criminals. There is another organization that works to maintain peace throughout the world, and it is known as the Cipher Pol. It is a group of secret agencies that directly operate under the command of the World Government. The two organizations are vastly different in structure and duties. Given the recent developments in the story, there is undeniably some kind of tension between the two.
The Marines
Out of the Three Great Powers, the Marines have the most manpower. There are thousands of soldiers, who serve in the Marines. At the top of the Marines is the Fleet Admiral, and currently, the position is held by Akainu. Below the Fleet Admiral are the three Admirals, who are considered to be the biggest strength of the Marines. Any person serving in either of these two positions seems to have an immensely powerful devil fruit. The next highest is the rank of Vice Admiral, followed by Rear Admiral, and then Commodore.
The Marines do their work publicly, and they are not as reserved as the Cipher Pol. While having a great deal of manpower definitely has its pros, it also has its downsides. The massive manpower that they have at their disposal means that there is more chance of information being leaked and someone infiltrating their ranks. A good example is the case of Vergo, who was serving in the Marines only to provide information to his boss, Doflamingo. In order to combat this issue, the Marines came up with a secret organization known as SWORD. There is not much information available about the goals of this organization except that they have been trying to monitor the Yonko. A few months ago, it was revealed that SWORD has a rivalry with CP0. If they are working towards the same goal of eradicating pirates, it doesn't make sense for these two organizations to harbor animosity towards each other.
The Infamous Cipher Pol
Cipher Pol is actually a string of different agencies. They are under the direct command of the World Government. They are mainly used for assassinations and investigations. There are a total of 10 Cipher Pols divisions; eight of them are officially recognized and two of them are not. The official Cipher Pol units range from one to eight, and the unofficial ones are CP0 and CP9. Both CP0 and CP9 enjoy certain privileges that put them above the other Cipher Pols and the Marines. CP9 members are allowed to kill any person who tries to interfere with their missions, and because of this reason, the existence of CP9 has been kept hidden from the public. CP0, on the other hand, is publicly known, so they cannot risk healing anyone openly.
Nine out of the ten Cipher Pols are under the control of the World Government, but CP0 takes orders directly from the World Nobles. There is a hierarchy within the Cipher Pols and the lower the number, the more important the mission assigned to them. As the organization with the lowest number, CP0 has been given many sensitive tasks by the World Nobles. Most of the time, Cipher Pol members are simply briefed and told to complete the task. However, there are some CP0 members who have access to sensitive information, and they are mainly the ones with the masks. According to Robin, the strongest members of the CP0 are given masks to wear to make them stand out from the weaker ones.
Why They Are Rivals
By now, it has been established that CP0 and SWORD carry out their missions in different ways. CP0 agents are trained to be ruthless, while members of SWORD still have some morals. When Koby was kidnapped by the Blackbeard pirates, the other members were worried, which shows that they care about them. CP0 members are nothing more than loyalists who do the dirty deeds of the World Nobles. The primary goal of SWORD is yet to be disclosed, but considering they have some of the most rational Marines working in the group, it is plausible to assume that their goal is for the betterment of all the people around the world.
The rivalry between them stems from the difference in their ideologies, and it has already caused conflict before. Some of the Marines have openly disobeyed the orders from higher-ups and condemned the methods of the World Government. Both of these things are completely unthinkable for CP0 members. The members of SWORD care about the common people, while the CP0 do not. In the past, CP0 members have even killed criminals without asking any questions. When Koby went to capture Boa Hancock, he tried everything to avoid the loss of lives. Needless to say, when the time comes for SWORD members to reveal their actual goal, it will give fans an even better idea of the rivalry between the two groups.