1930's, the conception of the federal relationship which on the whole
prevailed with the Court was a competitive conception, one which
envisaged the National Government and the States as jealous rivals. To
be sure, we occasionally get some striking statements of contrary
tendency, as in Justice Bradley's opinion in 1880 for a divided Court in
the Siebold Case,[16] where is reflected recognition of certain results
of the War between the States; or later in a frequently quoted dictum by
Justice McKenna, in Hoke _v._ United States, in which the Mann White
Slave Act was sustained in 1913:

Our dual form of government has its perplexities, State and
Nation having different spheres of jurisdiction ... but it
must be kept in mind that we are one people; and the powers
reserved to the states and those conferred on the nation are
adapted to be exercised, whether independently or
concurrently, to promote the general welfare, material and
moral.[17]

The competitive concept is, nevertheless, the one much more generally
evident in the outstanding results for American Constitutional Law
throughout three-quarters of its history. Of direct pertinence in this
connection is the doctrine of tax exemption which converted federalism
into a principle of private immunity from taxation, so that, for
example, neither government could tax as income the official salaries
paid by the other government.[18] This doctrine traces immediately to
Marshall's famous judgment in McCulloch _v._ Maryland,[19] and bespeaks
a conception of the federal relationship which regards the National
Government and the States as bent on mutual frustration. Today the
principle of tax exemption, except so far as Congress may choose to
apply it to federal instrumentalities by virtue of its protective powers