What responsibility does the federal government have to the states?

For the answer to the question above, one of the federal government's most important responsibilities to the states is Defense. The Constitution grants the Federal Government the authority to raise an Army to fight wars on land and a Navy to protect American shipping. When the Barbary Pirates were hurting American shipping and costing a fortune, the Navy had to attack the pirate headquarters in Tripoli. They needed a Land Arm, so the Marine Corps was instituted. (Actually, it had a very brief life during the Revolution, but was disbanded very shortly after). That's why the Marine Hymn says "To the shores of Tripoli". 

Of course, there were no planes back when the Constitution was written, but the Army formed an Air Corps during WWI, which fought in that war and in WWII. Shortly after WWII, the Army Air Corps became the Air Force.

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