The United States of America (USA), commonly referred to as the United States (US), America, or simply the States, is a federal republic consisting of 50 states, 16 territories, a federal district, and various overseas extraterritorial jurisdictions. The 48 contiguous states and the federal district of Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. At 3.79 million square miles (9.83 million km2) in total and with around 316 million people, the United States is the fourth-largest country by total area and third largest by population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The geography and climate of the United States is also extremely diverse, and it is home to a wide variety of wildlife.
Paleo-indians migrated from Asia to what is now the US mainland around 15,000 years ago, with European colonization beginning in the 16th century. The United States emerged from 13 British colonies located along the Atlantic seaboard. Disputes between Great Britain and these colonies led to the American Revolution. On July 4, 1776, delegates from the 13 colonies unanimously issued the Declaration of Independence. The ensuing war ended in 1783 with the recognition of independence of the United States from the Kingdom of Great Britain, and was the first successful war of independence against a European colonial empire. The current Constitution was adopted on September 17, 1787. The first 10 amendments, collectively named the Bill of Rights, were ratified in 1791 and guarantee many fundamental civil rights and freedoms.
A police officer (also known as a policeman, police agent, patrolman, cop, policewoman, and constable in some forces, particularly in the United Kingdom and other Commonwealth nations) is a warranted employee of a police force. In the United States, "officer" is the formal name of the lowest police rank. In many other countries, "officer" is a generic term not specifying a particular rank, and the lowest rank is often "constable". In many other countries there is no such title as "police officer", as the use of the rank "officer" is legally reserved for military personnel only and thus not applicable. Police officers are generally charged with the apprehension of criminals and the prevention and detection of crime, and the maintenance of public order. Police officers may be sworn to an oath, and have the power to arrest people and detain them for a limited time, along with other duties and powers.
Some police officers may also be trained in special duties, such as counter-terrorism, surveillance, child protection, VIP protection, and investigation techniques into major crime, including fraud, rape, murder and drug trafficking.
National security is the requirement to maintain the survival of the state through the use of economic power, diplomacy, power projection and political power. The concept developed mostly in the United States after World War II. Initially focusing on military might, it now encompasses a broad range of facets, all of which impinge on the non military or economic security of the nation and the values espoused by the national society. Accordingly, in order to possess national security, a nation needs to possess economic security, energy security, environmental security, etc. Security threats involve not only conventional foes such as other nation-states but also non-state actors such as violent non-state actors, narcotic cartels, multinational corporations and non-governmental organisations; some authorities include natural disasters and events causing severe environmental damage in this category.
Measures taken to ensure national security include: