The United States Armed Forces are the military forces of the United States. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard. The United States has a strong tradition of civilian control of the military. The President is the overall head of the military, and helps form military policy with the United States Department of Defense (DoD), a federal executive department, acting as the principal organ by which military policy is carried out. The DoD is headed by the Secretary of Defense, who is a civilian and Cabinet member. The Secretary of Defense is second in the military's chain of command, just below the President, and serves as the principal assistant to the President in all matters related to the Department of Defense. To coordinate military action with diplomacy, the President has an advisory National Security Council headed by a National Security Advisor. Both the President and Secretary of Defense are advised by a seven-member Joint Chiefs of Staff, which includes the head of each of Department of Defense service branches as well as the chief of the National Guard Bureau. Leadership is provided by the Chairman of the Joint Chiefs of Staff and the Vice Chairman of the Joint Chiefs of Staff. The Commandant of the Coast Guard is not a member of the Joint Chiefs of Staff.

The armed forces of a country are its government-sponsored defense, fighting forces, and organizations. They exist to further the foreign and domestic policies of their governing body, and to defend that body and the nation it represents from external and internal aggressors. In some countries paramilitary forces are included in a nation's armed forces. Armed force is the use of armed forces to achieve political objectives.

The study of the use of armed forces is called military science. Broadly speaking, this involves considering offense and defense at three "levels": strategy, operational art, and tactics. All three levels study the application of the use of force in order to achieve a desired objective.

In most countries the basis of the armed forces is the military, divided into basic Armed services. However, armed forces can include other paramilitary structures. For example, according to the Lithuanian law on organisation of system of national defence and military service (version actual from November 11, 2004), the Lithuanian armed forces in case of war also include border guards, public police service, parts of Lithuanian Riflemen's Union and guerrillas.

The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.