There are many factors that contributed to the creation of modern American police institutions. Many early American colonies had some form of law enforcement, usually in the form of Night Watches or Slave Patrols. The Night Watches' principal duties were to guard large cities against fires, crime, and disorder. Since it was a principally agrarian society at this time, the colonies adopted a simple law enforcement system in which governors would appoint a sheriff to oversee the law enforcement of a certain region, with the general public electing a constable, whose principal duty in New England was to control the Native American population. Aside from the aforementioned duties, the Night Watches were also relied upon to deal with the questions of controlling Natives and African-Americans to help keep the peace.

The organizations known as Night Watches, which would eventually mold into the Police Departments, developed mostly in Northern colonies like Massachusetts. The South also had their own form of early law enforcement. There, volunteer groups of white men formed Slave Patrols. The Carolina colony formed the first formal Slave Patrol in 1704. These Patrols were designed to control, punish and capture runaway slaves. These "Slave Patrols" are largely considered to be the first real police institution in the United States, due to the Northern Night Watches' lack of scope or power. They held so much power that Congress passed a law designed to make their jobs a national issue in the Fugitive Slave Act of 1850.

Throughout their existence, the organizations known as Slave Patrols existed largely as the arm of their regions' slave owners. They existed to return runaway slaves to their owners and to further the social and economic agendas of those slave owners. Like most facets of Southern society, law enforcement was another way for plantation owners to cement their control and dominance over the South and its people.

During the 1800s, the Slave Patrols and Night Watch organizations found themselves in a world that had advanced beyond them. Due to the new national focus on industrialism, hundreds of thousands of people migrated to major American cities. Dozens of different minority groups - such as African-Americans, Natives, Immigrants, and poor whites - were forced into small apartments and filthy living conditions. Partly as a response, these cities became places of incessant violence, and chaos. In response, the current law enforcement organizations morphed into the modern Police Departments we know today, They did this after seeing that cities had advanced beyond Slave Patrols and Night Watches, and seeing the results of similar changes in England. However, the Ku Klux Klan would soon come into existence to further the purposes and agendas of the Slave Patrols in the post-Civil War period.

Here are three separate articles I would recommend if you're interested in getting a little more in-depth with the research of early American law enforcement: http://law.jrank.org/pages/1640/Police-History-Early-policing-in-colonial-America.html

http://plsonline.eku.edu/insidelook/brief-history-slavery-and-origins-american-policing

http://ncpedia.org/slave-patrols

This is the Amazon page for "To Protect And Serve: A History of Police in America:"http://www.amazon.com/To-Protect-Serve-History-America/dp/0131120646 Its an interesting and informative read, if not a little expensive.