Command over the U.S. Armed Forces is established in the Constitution. The sole power of command is vested in the President by Article II as Commander-in-Chief. The Constitution presumes the existence of "executive Departments" headed by "principal officers", whose appointment mechanism is provided for in the Appointments Clause. This allowance in the Constitution formed the basis for creation of the Department of Defense in 1947 by the National Security Act. The DoD is headed by the Secretary of Defense, who is a civilian and member of the Cabinet. The Defense Secretary is second in the U.S. Armed Forces chain of command, with the exception of the Coast Guard, which is under the Secretary of Homeland Security, and is just below the President and serves as the principal assistant to the President in all defense-related matters. Together, the President and the Secretary of Defense comprise the National Command Authority, which by law is the ultimate lawful source of military orders.
All of the branches work together during operations and joint missions in Unified Combatant Commands, under the authority of the Secretary of Defense with the typical exception of the Coast Guard. Each of the Unified Combatant Commands is headed by a Combatant Commander, a senior commissioned officer who exercises supreme command authority per 10 U.S.C. § 164 over all of the forces, regardless of branch, within his geographical or functional command. By statute, the chain of command flows from the President to the Secretary of Defense to each of the Combatant Commanders. In practice, the Chairman of the Joint Chiefs of Staff often acts as an intermediary between the Secretary of Defense and the Combatant Commanders.
As of 2017, the U.S. spends about US$610 billion annually to fund its military forces and Overseas Contingency Operations. Put together, the U.S. constitutes roughly 40 percent of the world's military expenditures. The U.S. Armed Forces has significant capabilities in both defense and power projection due to its large budget, resulting in advanced and powerful technologies which enables a widespread deployment of the force around the world, including around 800 military bases outside the United States. The U.S. Air Force is the world's largest air force, the U.S. Navy is the world's largest navy by tonnage, and the U.S. Navy and the U.S. Marine Corps combined are the world's second largest air arm. In terms of size, the U.S. Coast Guard is the world's 12th largest naval force. 
The obligations of military employment are many. Full-time military employment normally requires a minimum period of service of several years; between two and six years is typical of armed forces in Australia, the UK and the US, for example, depending on role, branch, and rank. Some armed forces allow a short discharge window, normally during training, when recruits may leave the armed force as of right. Alternatively, part-time military employment, known as reserve service, allows a recruit to maintain a civilian job while training under military discipline at weekends; he or she may be called out to deploy on operations to supplement the full-time personnel complement. After leaving the armed forces, recruits may remain liable for compulsory return to full-time military employment in order to train or deploy on operations.
In the Navy, the rank of Admiral of the Navy theoretically corresponds to that of General of the Armies, though it was never held by active-duty officers at the same time as persons who held the rank of Fleet Admiral. George Dewey is the only person to have ever held this rank. After the establishment of the rank of Fleet Admiral in 1944, the Department of the Navy specified that the rank of Fleet Admiral was to be junior to the rank of Admiral of the Navy. However, since Dewey died in 1917 before the establishment of the rank of Fleet Admiral, the six-star rank has not been totally confirmed.
There are several different authorized pay grade advancement requirements in each junior-enlisted rank category (E-1 to E-3), which differ by service. Enlistees in the Army can attain the initial pay grade of E-4 (specialist) with a four-year degree, but the highest initial pay grade is usually E-3 (members of the Army Band program can expect to enter service at the grade of E-4). Promotion through the junior enlisted ranks occurs after serving for a specified number of years (which can be waived by the soldier's chain of command), a specified level of technical proficiency or maintenance of good conduct. Promotion can be denied with reason.
From the time of its inception, the U.S. Armed Forces played a decisive role in the history of the United States. A sense of national unity and identity was forged as a result of victory in the First Barbary War and the Second Barbary War. Even so, the founders of the United States were suspicious of a permanent military force. It played a critical role in the American Civil War, continuing to serve as the armed forces of the United States, although a number of its officers resigned to join the military of the Confederate States. The National Security Act of 1947, adopted following World War II and during the Cold War's onset, created the modern U.S. military framework. The Act established the National Military Establishment, headed by the Secretary of Defense; and created the Department of the Air Force and the National Security Council. It was amended in 1949, renaming the National Military Establishment the Department of Defense, and merged the cabinet-level Department of the Army, Department of the Navy, and Department of the Air Force, into the Department of Defense.
In 1974, the first six women naval aviators earned their wings as Navy pilots. The Congressionally mandated prohibition on women in combat places limitations on the pilots' advancement, but at least two retired as captains. In 1989, Captain Linda L. Bray, 29, became the first woman to command American soldiers in battle during the invasion of Panama. The 1991 Gulf War proved to be the pivotal time for the role of women in the U.S. Armed Forces to come to the attention of the world media; there are many reports of women engaging enemy forces during the conflict.
Additionally, many ASVAB practice tests have a section explaining the answer choices. It can be tempting to read the explanation and think that you now have a good understanding of the concept. However, an explanation likely only covers part of the question’s broader context. Even if the explanation makes sense, go back and investigate every concept related to the question until you’re positive you have a thorough understanding.