To be sure, this was only a means of checking state action, but from the doctrine of federal supremacy it logically follows that it is not within the power of a state to punish acts done under authority of federal law. At the time the question of incendiary publications was acute, the Supreme Court had not decided the line of cases upholding the right of removal to federal courts and sanctioning the release of officers for acts done in pursuance of federal authority. These cases declared it to be “an incontrovertible principle that the government of the United States may, by means of physical force exercised through its agents, execute on every foot of American soil, the powers and functions that belong to it. This necessarily involves the power to command obedience to its laws, and hence the power to keep the peace to that extent. This power to enforce its laws and to execute its functions does not derogate from the power of the states to execute its laws at the same time and in the same places. The one does not exclude the other, except where both cannot be executed at the same time. In that case the words of the Constitution itself show which is to yield. ‘This Constitution, and all laws which shall be made in pursuance thereof ... shall be the supreme law of the land.’”[390] And on the basis of this principle, there is no reason to hold that the postal employees could not be punished for distributing the incendiary matter when it was their federal duty so to do. To be sure, as urged by Cushing and Tucker, the United States guarantees each state a republican form of government and protects it against domestic violence, but this does not mean that a law which is passed by Congress to apply uniformly to the whole country, and which may, on account of peculiar local conditions, aid insurrectionary movements in certain of the states, is thereby unconstitutional. The resort of the states is not to the courts, but to Congress for the repeal of the harmful measure. Furthermore, the guarantee does not obligate the United States to insure a state against the occurrence of any violence, but simply to protect it when the violence is attempted. Since, therefore, the federal laws made criminal the detention of any mail matter, with only such exceptions as Congress might introduce, there was no way in which the states might enforce their laws against incendiary literature, unless they could exclude it absolutely from their borders.