United States Armed Forces
The United States Armed Forces make up America's military.
 The Army
The United States Army is a branch of the armed forces of the United States (the other four are the Navy, the Air Force, the Marines, and the Coast Guard, leading to the symbolism of the shape of their administration building, the Pentagon). The US Army is the oldest of these five branches, and used to have its own cabinet department (the Department of War). Now, however, its administration falls under the auspices of the Department of Defense.
The US Army is still the largest branch of the United States' armed forces, though it tends to get the most ridicule from the other branches. Also, despite having more personnel than either the USAF or USN, it receives nearly the same amount of funding. If Air Force or Navy personnel are ever forced to stay on an Army base, they are compensated for the "poor living conditions."
It was established on June 14, 1775 under the guise of the Continental Army. It is very proud of its traditions (though it is not nearly as tradition-bound as the US Navy). The ethos that has guided the army for most of its history is that of the "citizen-soldier," and it likes to consider itself the most egalitarian of the branches. Which is why it has the lowest standards for joining.
It should be noted that, despite the general perception of intellectual inferiority in the Army, some of the most educated and most influential officers of the modern era (such as General Petraeus and Colonel Gentile) are Army officers.
Due to being constantly understaffed, it is often the easiest branch to get into. In particular, the Army is often more flexible with fitness guidelines than the other branches. Despite this, roughly 27% of young Americans are still too obese to join.
Founded on October 13, 1775, the USN was the Yanks' response to the British Royal Navy. Hopelessly outmanned, outgunned, and outfunded, the Americans went to their crepe-eating cousins for help. This was the last time the French helped anyone (except Argentina during the Falklands War).
The years since then have seen the force increase from basically a couple of near-pirates to the world's strongest. A good example of the current strength of the Navy can be seen in the number of aircraft carrier groups that are deployed by different countries, as aircraft carriers are the capital ships of the day. They are the greatest projection of force on the water today. China does not have one, Russia still has one from the old Soviet Union, and France has one. The US, by contrast, has eleven. The US maintains a similar advantage in submarines (both ballistic and attack), VSTOL carriers (used most for helicopters), and almost every other kind of ship.
 Rank Structure
The US Navy employs a few people that run around and bark orders at their minions. Despite having really cool-looking officers' uniforms, US Naval Officers aren't as cool as the Marines. They are noted for pronouncing 'lieutenant' completely wrong.
 Warrant Officers
Almost there, but not quite.
Despite having really gay uniforms, enlisted folks in grades E-1 through E-6 work really hard at combating the stereotype produced by The Village People. Those lucky enough to make the cut to the Chief Petty Officer ranks (E-7 through E-9) get to sit in the goat locker.
 The Marines
The United States Marine Corps is one of five branches of the Armed Forces. Although a separate service, it is a department under the United States Navy. It is the mens' department. Originally called the Continental Marines, it was formed in 1775 in a bar called Tun Tavern.
The mission of the Marine Corps is to "wage and win wars.." against all the
godless, liberal freaks of the world enemies of freedom and democracy, and invade their countries, kill their leaders and convert them to Christianity.
 Air Force
The United States Air Force was originally part of the Army before becoming its own branch in 1947. It has over 300,000 active duty members and is headed by the Secretary of the Air Force and is a part of the Department of Defense. The United States Air Force Academy is located in Colorado Springs, Colorado.
The mission of the U.S. Air Force has shifted over the years, since the Army, Navy, and Marines all have their own aircraft. During the Cold War, the Air Force was home to the Strategic Air Command, which was basically the bombers and (land-based) ICBMs capable of waging nuclear war. Today, they run the strikefighters that operate from land-based airstrips.