It follows that the two governments, national and state, must each exercise its powers so as not to interfere with the free and full exercise by the other of its powers. To do otherwise would be contrary to the fundamental compact embodied in the Constitution--in other words, it would be _unconstitutional_. This proposition was affirmed at an early day by Chief Justice John Marshall in the great case of _McCulloch vs. The State of Maryland_,[1] which involved the attempt of a state to tax the operations of a national bank. That case is one of the landmarks of American constitutional law. While it did not expressly decide that the Federal Government could not tax a state instrumentality but only the converse, i.e., that a state could not tax an instrumentality of the nation, the Court has held in many subsequent decisions that the proposition enunciated by the great Chief Justice works both ways. For example, it has declared that a state cannot tax the obligations of the United States because such a tax operates upon the power of the Federal Government to borrow money[2] and conversely, that Congress cannot tax the obligations of a state for the same reason;[3] that a state cannot tax the emoluments of an official of the United States[4] and conversely, that the United States cannot tax the salary of a state official;[5] that a state cannot impose a tax on the property or revenues of the United States[6] and conversely, that Congress cannot tax the property or revenues of a state or a municipality thereof.[7] [Footnote 1: 4 Wheaton, 316.] [Footnote 2: _Weston v. City of Charleston_, 2 Pet., 449.] [Footnote 3: _Mercantile Bank v. New York_, 121 U.S., 138, 162.] [Footnote 4: _Dobbins v. Commissioner of Erie County_, 16 Pet., 435.] [Footnote 5: _Collector v. Day_, 11 Wall., 113.] [Footnote 6: _Van Brocklin v. Tennessee_, 117 U.S., 151.] [Footnote 7: _United States v. Railroad Co._, 17 Wall., 322.] The Supreme Court has said (and many times reiterated in substance) that the National Government "cannot exercise its power of taxation so as to destroy the state governments, or embarrass their lawful action."[1] One of the most distinguished writers on American Constitutional law (Thomas M. Cooley, Chief Justice of the Supreme Court of Michigan and afterward Chairman of the federal Interstate Commerce Commission) has said: There is nothing in the Constitution which can be made to admit of any interference by Congress with the secure existence of any state authority within its lawful bounds. And any such interference by the indirect means of taxation is quite as much beyond the power of the national legislature as if the interference were direct and extreme.[2] [Footnote 1: _Railroad Co. v. Peniston_, 18 Wall., 5, 30.] [Footnote 2: _Cooley's Constitutional Limitations_, 7th Ed., 684.]