Among the express limitations on the powers of the state which were incorporated into the federal constitution in 1787, were prohibitions against entering into any treaty, alliance, or confederation, granting letters of marque and reprisal, keeping troops or ships of war in time of peace, entering into any agreement or compact with another state or with a foreign power, or engaging in war unless actually invaded, or in such imminent danger as will not admit of delay (Art. I, § 10). The object of the prohibition as to treaties, alliances, or confederations among themselves, or with a foreign power, was no doubt to prevent the states from having any relations with each other as independent powers other than expressly authorized by the constitution; and also to prevent any attempt on the part of one state to obtain an advantage over another by arrangements, commercial or otherwise, with foreign countries. The broad principle was established that as to the relations of the states among themselves and as to the relations of the people of the United States to those of foreign countries, the federal government should be supreme and its power exclusive. But evidently an important consideration was to avoid hostility between the states, and alliances with foreign governments which might involve offensive or defensive military operations, and this idea is carried out in the prohibition as to maintaining troops or ships of war and engaging in war.

The war power, in a broad sense, is reserved to the federal government. But a state may, in case of sudden emergency, repel invasion, and in doing so it may perhaps carry on military operations outside its own boundaries, such operations being limited, however, to the purpose of preventing an imminent invasion.