The origination has already been touched on so I will not belabor the point.
As to why they currently are separated I can only tell you why the USMC thinks so.
The Marine Corps exists because the people of the US want it to. Plain and simple. The Marine Corps understands that as soon as the American People don't want a Marine Corps it will cease to be.
There are lots of arguments for keeping the Corps but I want to play Devil's Advocate here.
Who has made the largest most successful Amphibious Assault in history?
US Army at Normandy.
Who has better air to air combat Pilots?
US Air Force
Who has the ships and fights the best at sea?
Who has the best theme song!?
From Wikipedia, the free encyclopedia
The "Marines' Hymn" is the official hymn of the United States Marine Corps. It is the oldest official song in the United States military. The song has an obscure origin‚ÄĒthe words date from the 19th century, but no one knows the author. The music is from the Gendarmes' Duet from the opera Genevi√®ve de Brabant by Jacques Offenbach, which debuted in Paris in 1859. The Marine Corps secured a copyright on the song on August 19, 1919, but it is now in the public domain.
The initial verse is "From the Halls of Montezuma to the Shores of Tripoli." "Montezuma" refers to the Battle of Chapultepec, more commonly known as the battle which took place during the Mexican-American War; "Tripoli" refers to the First Barbary War and the Battle of Derne.
The "Marines' Hymn" is typically sung at the position of attention as a gesture of respect. However, the third verse is also used as a toast during events important to the Corps such as the Marine Corps birthday, promotions, and retirements. Note the line "Here's health to you and to our Corps."
From the Halls of Montezuma,
To the shores of Tripoli;
We fight our country's battles
In the air, on land, and sea;
First to fight for right and freedom
And to keep our honor clean;
We are proud to claim the title
Of United States Marines.
Our flag's unfurled to every breeze
From dawn to setting sun;
We have fought in ev‚Äôry clime and place
Where we could take a gun;
In the snow of far-off Northern lands
And in sunny tropic scenes;
You will find us always on the job
The United States Marines.
Here's health to you and to our Corps
Which we are proud to serve;
In many a strife we've fought for life
And never lost our nerve;
If the Army and the Navy
Ever look on Heaven‚Äôs scenes;
They will find the streets are guarded
By United States Marines.
And who has the best epitaph?
"And when he reaches Heaven,
to Saint Peter he will tell,
'Another Marine reporting, sir...
I've served my time in hell"