I've gotten questions from people around the world about the US military. Even as a citizen, I admit that it can be kind of confusing. This post is meant as a brief guide to the layout of the US armed forces.

Branches of the United States Department of Defense:
The Department of the Army - exactly what it sounds like
The Department of the Air Force - exactly what it sounds like
The Department of the Navy - the Navy and also the Marine Corps

Yes, both the Navy and Marines are under the Department of the Navy. The top Marine and top Sailor both report to the Secretary of the Navy, who along with the Secretaries of the Army and Air Force report to the Secretary of Defense, who reports to the President. (organizational chart here)

This generally trips people up because many countries don't have a Marine Corps, so let me go into more detail. The Marine Corps started out in the 1700's doing boarding parties and sharpshooting during ship-to-ship battles, as well as ship-to-shore landings. They were basically naval infantry. Over time, they grew more diverse and became a separate service, but are still closely tied to the Navy.

Note: "Corps" is pronounced "core" not "corpse." It's one of those Latin things.

So, what do each of the branches - Army, Marine Corps, Navy, Air Force - bring to the table?

Initially, the Air Force seems pretty simple. They have the best bombers, fighters, transports, and helicopters.
But: They also do nuclear ballistic missiles, cyberspace, Pararescue behind enemy lines, global surveillance, and space operations.

Okay, so the Air Force is more than meets the eye. What about the Army? They certainly do a lot of ground operations, special forces, and whatnot.
But: The Army has its own helicopters for attack and transport, engineers to build things, missile defense, and believe it or not more small boats than the Navy.

The Navy is known for having ships, submarines, and everything else nautical.
But: They also have their own air force. While they don't have large bombers, they do have transports, helicopters, and fighters. In fact, they have more planes than the actual Air Force. The Navy also has nuclear ballistic missiles on submarines, its famous SEALs, and Seabee ground engineering/building units.

Want to muddy the picture even further? Most of the movies you see about Marines depict them as infantry.
But: They have a small air force too, with helicopters, transports, and fighters. They also regularly train for amphibious operations. Their focus is rapid deployment anywhere, anytime. They are the smallest force and generally don't have the best equipment, but they are good at multitasking and rapid deployment.

So to recap:
Army is excellent for land
Marines are good for land and air, and good at working with the Navy
Navy is excellent for sea and good for air
Air Force is excellent for air
And like any good sibling rivalry, they sometimes get up in each other's stuff

So that was terribly simplified and I've probably left stuff out but it should give a decent idea of the big picture.
Questions/comments? Maybe this can be expanded to include general armed forces information for writers who want to know.

Uh, with my limited knowledge, I can't explain everything but some things you correct about, I'll ask my dad, he was a seal in the 80's he would know... I'll be back!

1930898 ignoring the ponies, he said your spot on, with everything.

1930908 It's what I do.

What about the Coast Guard? Aren't they part of the armed forces?

1930963 In wartime the Coast Guard can be commanded by the Department of the Navy. Usually, they're part of the Department of Homeland Security and act as law enforcement.

1930879 Very informative post. So what about amphibious assault ships and landing craft? Who operates them?

1933505
For the most part it's the Navy, though the Army does have it's own landing craft

1933505>>1933570 By "amphibious assault ship" I assume you mean one of these babies. The Navy handles the transportation, like a taxi service for the Marines.

Rule of thumb - if it's Ship, it's probably Navy. If it's a Boat, it could be someone else.
But hovercrafts and most landing craft are Navy.

1933788
I didn't say that the Army had their own Amphibs, and they do have their own landing craft, the Runnymede-class, 35 of them to be exact.

1930879
A nice summary overall.

The only thing I want to say is that I don't think the Air Force has the best helicopters (It hurts to admit it); i'd have to say the Army operates the best, as the AF doesn't even have any direct-combat helos and each branch has their own for Combat, Support, and other tasks. The best in the AF inventory is the Pave Hawk.

Also, just for future readers, you might want to add a little paragraph on how each has their own personal Spec. Ops. units (such as Delta-Army, SEAL-Navy, Pararescue-AF) to avoid people from thinking they all serve in one super-secret special branch (I've had a few ask me that before) or misidentifying them.

1933925 The SF thing gets even more complicated when you remember the Army has Green Berets, Rangers, and Delta. If you also include SOAR, then the Navy will probably argue that their SWCC and SEAL delivery guys are SF. The Coast Guard technically has its own group. Not to mention CIA SAD.

You forgot SOCOM and JSOC

1934846
And along with others like ISA, 160th, and Marine Force Recon. CIA SAD could be stuck in a branch all it's own, as it's broken down into groups like SOG and PAG. Even with all that, there are people that'll come up asking about groups that don't exist any more, like the OSS.

If I had to break it down, i'd follow the Tier procedure for Special Missions that's in place. Black ops units like Delta, SEAL, and SAD seem to do the most secretive missions, such as raids and assassinations. Special Forces are the Green Berets, Rangers, and 101st and handle the more battlefield operations (not to say the others won't but that these specialize in them) like launching offensives against known enemy strongholds.

Out of all of these, I want to say the Rangers have been the most effective over the course of it's existence.

1935738 To be fair, if we were going to cover SOCOM, we'd also have to do the other Unified Combatant Commands: STRATCOM, TRANSCOM, EUCOM, NORTHCOM, SOUTHCOM, AFRICOM, PACOM, and CENTCOM. Can't have one without the others. As for JSOC, it's a subdivision of SOCOM.