Congress is expressly given power to declare the punishment of treason against the United States (Art. Ill, § 3), and in the same section of the constitution the crime of treason against the United States is expressly defined by the declaration that it "shall consist only in levying war against them [the United States], or in adhering to their enemies giving them aid and comfort." It is not usual expressly to define specific crimes by constitutional provisions, but historically there is a good reason for giving in state and in federal constitutions an explicit definition of this crime, so that the legislative power cannot, by statute, define treason so as to include any other acts than those enumerated. By the old English law, the crime of treason was divided into (1) petit treason, a crime against a superior, and (2) high treason directed against the sovereign or the government, and it is the latter offence which is referred to when the term "treason " is used in the constitution or statutes of the United States or the states.

By the early law of England, many acts were deemed high treason which were only constructively or inferentially offences against the sovereign or government, and the courts, by arbitrary construction, punished persons for treason who had not attempted any act directed against the sovereign or calculated to overthrow the government. Criticism of the sovereign or the government tending to lessen the public respect for either might be thus punished; and prosecutions for treason were resorted to for the purpose of intimidating or overthrowing those who were hostile to the ruler, although such hostility was not manifested by any act of violence. As early as the year 1350, English statutes were passed to remedy this abuse by specifically defining what should constitute high treason, and such statutory provisions have been in force in England to the present time. It is natural, therefore, that these provisions intended as a guaranty of the security of the subjects of England against the exercise of tyrannical power on the part of the government through the courts, should be embodied in substance in the state and federal constitutions; and the definition incorporated into the federal constitution is a portion of the definition found in the early English statutes. Under such constitutional provisions neither Congress nor a state legislature can make a definition for the crime of treason so as to include any acts not included in the terms of the federal or state constitution.

The levying of war, under the definition of treason against the United States, implies an assembling of persons with force and arms to overthrow the government or resist the laws. All who aid in the furtherance of the common object of levying war in however minute a degree or however remote they may be from the scene of action are guilty of treason. The enemies of the United States, within the language of the definition relating to lending aid and comfort to such enemies, must be those who are engaged in carrying on hostilities against the government. A mere conspiracy or intent to overthrow the government or to interfere with its operations, or an interference with the officers or agents of the government in the discharge of their duties, but not in pursuance of any general plan to resist the enforcement of the laws, will not constitute treason, although such an act may be punishable as constituting a crime of some other description.

During the war of the rebellion, it was held that the confederate government was for the time being a government waging war against the United States in such sense that participation in such war in hostility to the United States, or the act of adhering to such hostile government, rendering aid and comfort to it or its forces in the war, constituted treason against the United States (United States v. Greathouse). It is to be noticed, however, that acts of hostility on the part of the subject of a foreign government owing no allegiance to the United States by reason of citizenship or permanent or temporary residence within its limits cannot constitute treason. Such persons would not be subject to the laws of the United States. The citizens of the confederate states who engaged in rebellion against the federal government were guilty of treason, because while citizens and subjects of the United States they levied war against the United States.

Although the various states of the Union are not in every sense sovereign powers, nevertheless it seems to be conceded that treason may be committed as against a state and punished by the state as a crime. As a matter of fact, during the entire existence of the United States as a nation there have been very few prosecutions for treason either against the federal government or against a state.