Reading through the Avatar thread, I came across this: My question isn't about why the different branches of the military is a bit (or more) competitive with each other and all those things, but it got me wondering about something: Why the Marines? I mean, historically, there were warriors/soldiers to wage wars. And as we got the ability to cross water in boats, fighting necessarily followed. And since armed and armored soldiers don't swim very well, and boats can't sail over land, we got a military divided into two branches: the Army and the Navy. Fair enough. Then later, we got airplanes. And after a bit back and forth (still hasn't been completely settled as far as I know), we got a third branch: the Airforce. Or/and the Army and Navy have their separate air forces in some organisations. And then several countries have the Marines. Historically, I understand that they were assigned to ships to make sure that no sailors mutinied or something. More like a "supporting" branch than a real one in itself (but then again, that was also how the airforces started - or still is). But now - at least by looking at the USMC - they seem, from a quick glance, to be a second Army branch. And I believe they have their own air force as well? (Not to mention that the US and other countries have yet another branch - the National Guard - which seems like yet another Army branch to me. But that can be explained more as being a state level military instead of the federal, right?) Why? Why is the military divided into these branches? Is it simply by tradition and historical evolution, or is there a logic to it? Edit: And for one additional question: Are there better ways to organise a military than the currently most common one of Army, Navy, Airforce, Marines, National Guard?