I’m not from the US and have no military experience either. So the US has apparently 3 maritime branches in the uniformed services and the Coast Guard is, well guarding the coasts of the US. And the other branches can’t do that?
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The purpose of each are very different.
In most countries their Navy is what we term a green water navy, meaning it is largely designed for coastal water protection near the country’s own borders.
The United States Navy is not a green water navy. Its doctrine is what we call a blue water navy, where projection of force out into the world’s oceans is its mandate. The US navy secures shipping safety and regional stability the world over, thousands of kilometers from any port of call. The navy is a large scale fighting force, with most assets built around the idea that carrier battle groups are its primary means of projection, followed by missile destroyers and cruisers to counter high level warfare threats. High scale warfare against nation states, and their assets like submarines, missiles, and combat aircraft are their concern.
The navy has a secondary function in that is must support the mission of the marine corps as well, which is the force of ground combat designed to be landed on any beach on the planet inside 23 hours. They provide the close air support, logistics, and supply for landing amphibious invasions that the marine corps concerns itself with.
The marines are a rather specific force, designed primarily around coastal insertion and ground combat. They are intended to take the beaches and create landing areas for follow on invasion forces. The mission of the marine corps is summarized nicely in the motto of the marine corps rifle squad, “to close with and destroy the enemy by fire and close combat”. Marine corps units train as an offensive force that uses ferocity and overwhelming firepower to eliminate high level threats, like enemy state armies and combat forces. They operate 24 hours a day, 365 days a year with battalion sized armored light amphibious infantry units at sea in various parts of the world called marine expeditionary units (MEUs). These MEUs offer the first line of immediate regional stability and invasion need response. 3 divisions of marines exist in Okinawa, California, and North Carolina in active service with each focused on again large scale invasion and high level armed combat.
In each of these two branches missions, you note I said “high level conflict” a few times. That is a term designating military actions as separate from low level interdictions of border violations by smugglers, drug / human trafficking, security of ports and domestic water ways etc. These patrol duties are more policing on the water and a force designed to handle those low level conflicts and patrol security is the coast guard. They are not designed for combat and force projection, but instead for security and policing of local watersheds to handle border integrity as well as safety. When you are lost at sea, get carried out by a rip current, caught in a hurricane, or need ports protected for ship transit; the coast guard is there to help you.
So the first two are force projection high level conflict forces designed to destroy nation states and the coast guard is designed to be a low level conflict (interdicting drug boats mostly) force for policing and stability. With such widely different missions, you get widely different equipment needs and training needs.
It is generally good practice not to have people trained to destroy counties being in charge of policing local borders. The coast guard get smaller cutters with more utility and flexibility in the responsiveness they need to meet their mission, while the navy and marine corps get the sheer destructive firepower and high cost assets to destroy nation states overnight.
The US military has a very different requirement for its force projection than most any nation on earth, which may be part of the confusion for you coming from another country. The US military is considered only adequately prepared if it can, at any time, win a global war on the scale of full conflict with another peer superpower on the other side of the earth in one theater… and simultaneously HOLD a front against another global super power in another side of the world. That level of readiness and firepower require doctrine that is not compatible with low level coastal green water policing with its navy.
Hope that helps.
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