system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does the US Armed Forces have an Army, Navy, Marines, and Coast Guard, instead of just an Army and Navy.

It seems unnecessary to have all these branches when the Navy can just absorb the Coast Guard and the Army can absorb the Marines. To me it seems counter-intuitive for these specialized branches to exist when in reality they are essentially the same branch.

For example, Marines are supposed to specialize in invasion and ground combat, but so are Army Rangers. The Navy reigns in international waters but both the coast guard and the navy perform the same tasks with the same equipment.

To me, it seems expensive and overly complicated to keep 4 branches instead of 2.
gpt
Back when they first started making militaries, there were two kinds of fighting people:

* People who fought on land. When they got around to inventing the English Language, they called them *Armies*.

* People who fought on water, from ships. We call them *Navies*.

Now around the time of the American Revolutionary War (late 1770s), navies fought using wooden sailing ships with lots of cannons firing solid iron balls. These guns were pretty powerful, but they were pretty inaccurate unless you got really close, and unless you got really lucky and blew up the enemy's gunpowder magazine, they couldn't really do enough damage to sink another ship quickly. 

So if you were willing to sail through cannonball fire for a little bit, it was possible to get close enough to the enemy ship to board it with your own men. However, most sailors were good at sailing, and not as good at shooting or hand-to-hand fighting, so they decided to create groups of soldiers who specifically trained to fight from ships. These are called *Marines*. 

Some countries kept them as part of their armies or navies, but in the US, they put them specifically under the authority of the Department of the Navy, and slowly over time they gained more and various responsibilities- Presidential guard, Embassy guard, etc.  Also, because they were on Navy ships all the time, they were usually the first ground troops to show up when the government was trying to flex its power overseas, so they also started doing amphibious warfare (invading beaches from ships), which is their primary mission today.

Around the same time, many countries saw a need for a nautical police force. Using the Navy for that was in most cases overkill; you want to enforce tariffs and catch criminals, not blow merchant ships out of the water. So *Coast Guards* started becoming a thing, tasked with seagoing law enforcement and search and rescue. In the US, it was put under the authority of the Department of Transportation in peactime, and the department of the Navy in wartime. (there was no Department of Defense then). Recently they've been put under the Department of Homeland Security.

So, finally, years later, airplanes were invented. Immediately all the armed forces saw a use for them; mainly spotting for big guns at first (Navy battleships and Army artillery), but later they started carrying guns and bombs and torpedoes and missiles. In some countries, they managed to create a separate *Air Force* right away (the UK did this with the RAF), but in the US, the Army and Navy each handled their own aviation, arguing (accurately enough) their needs were different. So it wasn't until after World War II that the Air Force was split off from the Army into a final branch. The Army still maintains its helicopters for transport and close air support, and the Navy argues that aircraft carrier operations are too different from the land flying that Air Force personnel are used to, so they each have their own aircraft. The Coast Guard needs aircraft for long-range search and rescue. And the Marines have their own aircraft because they're supposed to be a self-contained, independent and fast-moving fighting force once the Navy drops them on the beach.

As for why they haven't merged them back? Politics, money, and tradition. There have been several attempts made--the Air Force argued in the 50's that nuclear weapons made all surface forces obsolete; the Army has tried to absorb the Marines several times; the Air Force has tried to take the Navy's planes, etc. But it seems unlikely in the near future.