Active duty in the context of Massachusetts State Militia

The United States Navy (USN) is the maritime service branch of the United States Armed Forces and the naval warfare force of the United States. It is the world's most powerful navy with the largest displacement, at 4.5 million tons in 2021. It has the world's largest aircraft carrier fleet, with eleven in service, one undergoing trials, two new carriers under construction, and six other carriers planned as of 2024. With 336,978 personnel on active duty and 101,583 in the Ready Reserve, the U.S. Navy is the third largest of the United States military service branches in terms of personnel. It has 299 deployable combat vessels and about 4,012 operational aircraft as of 18 July 2023. The U.S. Navy is a part of the United States Department of Defense and is one of six armed forces of the United States and one of eight uniformed services of the United States.

The United States Navy traces its origins to the Continental Navy, which was established during the American Revolutionary War and was effectively disbanded as a separate entity shortly thereafter. After suffering significant loss of goods and personnel at the hands of the Barbary pirates from Algiers, the United States Congress passed the Naval Act of 1794 for the construction of six heavy frigates, the first ships of the Navy. The United States Navy played a major role in the American Civil War by blockading the Confederacy and seizing control of its rivers. It played the central role in the World War II defeat of Imperial Japan. The United States Navy emerged from World War II as the most powerful navy in the world, succeeding the British Navy. The modern United States Navy maintains a sizable global presence, deploying in strength in such areas as the Western Pacific, the Mediterranean, and the Indian Ocean. It is a blue-water navy with the ability to project force onto the littoral regions of the world, engage in forward deployments during peacetime and rapidly respond to regional crises, making it a frequent actor in American foreign and military policy.

Commodore was an early title and later a rank in the United States Navy, United States Coast Guard and also has been a rank in the United States Public Health Service Commissioned Corps and the National Oceanic and Atmospheric Administration Commissioned Officer Corps (NOAA Corps) and its ancestor organizations. For over two centuries, the designation has been given varying levels of authority and formality.

Today, it is no longer a specific rank within active-duty or reserve forces or in the Public Health Service Commissioned Corps or NOAA Corps, but it remains in use as an honorary title within the U.S. Navy and U.S. Coast Guard for those senior captains (pay grade O-6) in command of operational organizations composed of multiple independent subordinate naval units, e.g., multiple independent ships, submarines, or aviation squadrons.

The Armed Forces of the Russian Federation, commonly referred to as the Russian Armed Forces, are the military of Russia. They are organized into three service branches—the Ground Forces, Navy, and Aerospace Forces—three independent combat arms (the Strategic Rocket Forces, Airborne Forces and Unmanned Systems Forces) and the Special Operations Forces Command.

The Russian Armed Forces are the world's fifth largest military force, with about one million active-duty personnel and close to two million reservists. They maintain the world's largest stockpile of nuclear weapons, possess the world's second-largest fleet of ballistic missile submarines, and are the only armed forces outside the United States and China that operate strategic bombers. As of 2024, Russia has the world's third-highest military expenditure, at approximately US$149 billion, or over seven percent of GDP, compared to approximately to US$86.5–$109 billion the year before.

Conscription, also known as the draft in American English, is the practice in which the compulsory enlistment in a national service, mainly a military service, is enforced by law. Conscription dates back to antiquity and it continues in some countries to the present day under various names. The modern system of near-universal national conscription for young men dates to the French Revolution in the 1790s, where it became the basis of a very large and powerful military. Most European nations later copied the system in peacetime, so that men at a certain age would serve 1 to 8 years on active duty and then transfer to the reserve force. In the early 2000, Norway and Sweden became the first nations to conscript women on the same legal terms as men. Denmark has committed to implement a similar system shortly.

Conscription is controversial for a range of reasons, including conscientious objection to military engagements on religious or philosophical grounds; political objection, for example to service for a disliked government or unpopular war; sexism, in that historically only men have been subject to the draft; and ideological objection, for example, to a perceived violation of individual rights. Those conscripted may evade service, sometimes by leaving the country, and seeking asylum in another country. Some selection systems accommodate these attitudes by providing alternative service outside combat-operations roles or even outside the military, such as siviilipalvelus (alternative civil service) in Finland and Zivildienst (compulsory community service) in Austria and Switzerland. Several countries conscript male soldiers not only for armed forces, but also for paramilitary agencies, which are dedicated to police-like domestic-only service like internal troops, border guards or non-combat rescue duties like civil defence.

The Ready Reserve is a U.S. Department of Defense program which maintains a pool of trained service members that may be recalled to active duty should the need arise. It is composed of service members that are contracted to serve in the Ready Reserve for a specified period of time as a reservist or in active duty status. Members of the Ready Reserve are required to be prepared for mobilization or re-activation within a specified period of time, maintain a serviceable uniform, and maintain a degree of fitness.

Each of the reserve components of the United States Armed Forces is divided into the Ready Reserve, Standby Reserve, and the Retired Reserve.

A military reserve force is a military organization whose members (reservists) have military and civilian occupations. They are not normally kept under arms, and their main role is to be available when their military requires additional manpower. Reserve forces are generally considered part of a permanent standing body of armed forces, and allow a nation to reduce its peacetime military expenditures and maintain a force prepared for war. During peacetime, reservists typically serve part-time alongside a civilian job, although most reserve forces have a significant permanent full-time component as well. Reservists may be deployed for weeks or months-long missions during peacetime to support specific operations. During wartime, reservists may be kept in service for months or years at a time, although typically not for as long as active duty soldiers.

In countries with a volunteer military, reserve forces maintain military skills by training periodically (typically one weekend per month). They may do so as individuals or as members of standing reserve regiments—for example, the UK's Army Reserve. A militia, home guard, state guard or state military may constitute part of a military reserve force, such as the United States National Guard and the Norwegian, Swedish and Danish Home Guard. In some countries (including Colombia, Israel, Norway, Singapore, South Korea, Sweden, and Taiwan), reserve service is compulsory for a number of years after completing national service. In countries with conscription, such as Switzerland and Finland, reserve forces are citizens who have completed active duty military service but have not reached the upper age limit established by law. These citizens are subject to mandatory mobilization in wartime and short-term military training in peacetime.

The chief of staff of the Army (CSA) is a statutory position in the United States Army held by a general officer. As the highest-ranking officer assigned to serve in the Department of the Army, the chief is the principal military advisor and a deputy to the secretary of the Army. In a separate capacity, the CSA is a member of the Joint Chiefs of Staff (10 U.S.C. § 151) and, thereby, a military advisor to the National Security Council, the secretary of defense, and the president of the United States. The CSA is typically the highest-ranking officer on active duty in the U.S. Army unless the chairman or the vice chairman of the Joint Chiefs of Staff are Army officers.

The chief of staff of the Army is an administrative position based in the Pentagon. While the CSA does not have operational command authority over Army forces proper (which is within the purview of the combatant commanders who report to the secretary of defense), the CSA does exercise supervision of army units and organizations as the designee of the secretary of the Army.

The chief of staff of the Air Force (acronym: CSAF, or AF/CC) is the service chief of the United States Air Force. They are the principal military advisor to the secretary of the Air Force on matter pertaining to the Air Force. They are a member of the Joint Chiefs of Staff, and thereby a military adviser to the National Security Council, the secretary of defense, and the president. The chief of staff is typically the highest-ranking officer on active duty in the Air Force, unless the chairman and/or the vice chairman of the Joint Chiefs of Staff are Air Force officers.

The chief of staff of the Air Force is an administrative position based in the Pentagon. The chief of staff does not have operational command authority over Air Force forces. That is within the purview of the combatant commanders who report to the secretary of defense. The chief of staff exercises supervision of Air Force units and organizations as the designee of the secretary of the Air Force.