To understand American law enforcement, you must understand that the United States is a federal republic. Power is shared between different levels of government, and each level of government is supreme within its area of influence. While the Constitution guarantees federal supremacy, it also limits the scope of the central government. Outside of that scope, the states are sovereign.

The states are generally responsible for most criminal justice. The state legislatures define most crimes. If a wife kills her husband, she will be tried under a state statute. 49 states have state police forces, but most of these are limited in their scope. Most are specialized agencies or they are agencies that will assist local law enforcement upon request.

Most actual law enforcement is handled by cities and counties (or parishes in the case of Louisiana). When a person calls for police assistance, it's a county deputy sheriff or a city police officer that will usually respond.

This system evolved partly from the tradition of county sheriff's elected at the county level. Also, the need for law enforcement has generally been felt first at a local level. In the 1800s, it was local communities that began appointing police officers. The division of police responsibility continues America's distrust of concentrated power. Agencies that have powers to arrest for any violation of the law have small geographical jurisdictions, whereas federal and state agencies with huge geographical areas, are limited in the laws that they can enforce.

So if the states write the laws and local governments enforce most of the laws, what role does the federal government have?

The federal government generally handles issues that are impractical for the states to handle. The federal government is basically responsible for foreign relations and interstate commerce. Over the years, it has taken on more responsibilities like Social Security and other social welfare programs. There are numerous federal law enforcement agencies that employ special agents, park rangers, police officers, inspectors, marshals, corrections officers, etc.

The federal government law enforcement comes from several sources. First off, the federal government has the right to protect itself from physical harm and fraud. So most federal agencies have some sort of law enforcement arm. These will generally be uniformed officers to protect their buildings, such as the Federal Protective Service or investigative agencies like the Offices of the Inspector General (OIG) which protect their agencies against fraud.

The federal government also has areas where it is responsible for law enforcement. They're responsible for collecting taxes, so there are several tax related agencies like the IRS . The feds are responsible for protecting our boarders so they formed the INS, the Coast Guard * and the Customs service.

Finally, there's the ubiquitous Federal Bureau of Investigations, much lauded in the press and much distrusted and disliked by every other law enforcement agency. They are responsible for every violation of federal law that isn't covered by another federal agency.