Local police forces emerged in the 19th century to address new challenges of social organisation brought on by urbanisation and industrialisation. However, while the police resolved new-found social problems, they also became, in some ways, a problem of their own. Since then, a debate has revolved over whether police forces should be locally or centrally controlled.
Centralised, as opposed to localised, police forces are more disengaged from the community. They may, in fact, not even reside anywhere near the community in which they work, and may have been posted their from a remote location elsewhere. Centralised police, therefore, may lack a feeling of solidarity of comradeship with the members of the community they are overseeing.
An advantage of a decentralised police force is local control. Citizens of a town, city, or community can exercise a greater degree of oversight and direction over their police. They have more say as to how the police operate and what aspects of public safety they focus on. This also raises the question of funding. Centralised police forces are funded by the central government. Local people, therefore, have no influence over how police funding is appropriated.
Decentralised police are empowered by and answer to centralised authorities. They have less accountability than do decentralised police. For example, many cities in the United States operate civilian review boards that oversee police action, investigate abuse, and issue recommendations or discipline. While such a board could theoretically exist at the national level, it would be free of local control and its members would be staffed by centralised, not local, representatives.
Exploitation by National Leaders
One underlying danger of a national police force is its vulnerability to being abused by the central government. Certainly it's true that decentralised police are also prone to committing abuses of power; in fact, it's a very real occurrence in many parts of the country. However, centralised police forces imbue the national government (and in particular a national executive) with a greater degree of power. A number of countries with centralised police forces (the former Soviet Union, South Africa, etc.) have in fact seen centralised police carry out abuse orders from the top down.