What was the basis for the court's ruling that Maryland could not tax the Second Bank of the United States

The ruling in the case of McCullough v. Maryland (1819) was based on the "necessary and proper" clause of Article I, Section 8 of the Constitution of the United States.

After enumerating a number of the powers of Congress, including borrowing money, coining money, regulating commerce, etc, Section 8 of Article I closes with by saying Congress shall have power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."

Alexander Hamilton had referenced the same constitutional clause when, as Secretary of the Treasury, he sought to establish the First Bank of the United States in 1791. A national bank was not strictly listed as something Congress could establish, but there was nothing in the Constitution to prohibit it. And the "necessary and proper" clause gave leeway to create such an institution. The First Bank of the United States existed from 1791 till 1811, when its charter expired and was not renewed by Congress. The Second Bank of the United States also existed for a 20-year charter period, from 1816 to 1846.