You're basically asking the question - why have a military at all? Our country plays a large part on the global stage (and I hope continues to play) and its critical that our military is second to none. I've never had second thoughts about our enormous defense spending nor the existence of our Navy and Air Force. I admit I've wondered what we need the Marines and USCG/USMM for. Simply because we haven't had a major air-to-air engagement since Vietnam and a major sea-to-sea engagement since WWII does not mean that the AF and the Navy are somehow useless. How do you know this won't be the case in this century or beyond?
Police officers work on a domestic scale, aren't employed by the federal government, nor are they responsible for protecting our national interests on a national or global scale. They are not responsible for the destruction of the enemies of the STATE. Maybe they're responsible for dealing with the guy who holds up a bank. Stopping crime. But thats not the same thing.
Right. Many people, as have already done, spout off the mission of their service or core values....same ole same ole. Reason for this....dig deeper.
Do we need a U.S. Marine Corps? Once we get through the oorah'ness of it all, why not have one land force, one sea and one air? Why have a district AF? With the exception of heavy lifters, bombers and tankers, what can an Air Force plane do that a Navy plane can't?
Why is the Navy investing so much in LCSs? Haven't we already seen in WWII that naval aviation is greater than any battle ship? What's a frigate do usually?
As people rattled off their branches reasons of being....it seemed to me MOST of them could have interchangeable service names in there.
Is that starting to make sense?
Why have a Coast Guard? That's a fine question to ask, especially if you're only looking at the military side of the coin. On the domestic side, the Coast Guard inspects ships, maintains all aids to navigation, runs dGPS, and of course provides port security, alien migrant interdiction, drug interdiction, and search and rescue. Not to mention, the Coast Guard has ice breakers.
Now, why military? A Coast Guardsmen will be the first to tell you that we're small, and so is our equipment. Coasties are expert small boat handlers. Better than any service. They are also very good at at sea boardings. Finally the Coast Guard is proficient in port security ops. Most of the navies in the world are far more similiar to the U.S. Coast Guard than the U.S. Navy. Because of the domestic missions, it also sometimes helps some countries feel a little more comfortable interacting with us. Finally, the Coast Guard is used to not only speaking "military" but also with civilian counterparts, something you soon come to find some branches struggle with from time to time. Why have a Merchant Marine? Well, most of the stuff used in a war isn't flown in. It's shipped.
Are there aspects of the Coast Guard that aren't very "military"? Sure. Small boat stations tend to be very domestic and LE-focused. That's largely because that's how U.S. Life Saving stations were created over a century ago. I know the average American would feel uncomfortable having Navy sailors pull them over to issue a ticket (of course, they can't do that though). Aids to navigation is the same way. Search and rescue would resemble any service SAR capabilities. I would say some of the large Coast Guard cutters are very much like their Navy sisters. There's little difference between a Navy frigate and a Coast Guard 378'. A lot of overlap between a Coast Guard NSC and a Navy LCS.
We, in the Coast Guard, are comfortable with our service, but sometimes have trouble explaining it to our sister services. I sometimes find it amazing that some services, like the Air Force, would have so much to say, but forget that they're 150 years younger than the Coast Guard. Or I'm surprised when a Marine says something, as apparently they forgot about the Coast Guardsmen who gave their lives protecting beached Marines in the Pacific in WWII. Certainly assume sailors who have something to say have never gone on a boarding at sea or worked with a LEDET. There's little interaction with Army, beyond their very limited transportation branch and their tugs (and I guess the RAID teams in Afghanistan and the PSUs in the NAG).