This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
Having shown the constitutional power of the United States to acquire territory whether by treaty, conquest, or discovery and occupation, we now approach the question as to the modes by which this federal authority may be exercised.
A history of the territorial expansion of the United States shows that territories have been annexed in three different ways: (1) by statute, (2) by treaty, and (3) by joint resolution.
The process of extending American sovereignty by simple statute and executive action authorized thereby was illustrated, as we have just seen, in the case of the Guano Islands. The annexation of territory by treaty has been the method most usually employed. The Louisiana Territory, Florida, Alaska, the Mexican cessions, the Samoan Islands, Porto Rico, and the Philippines were obtained in this manner. The constitutionality of this mode of acquisition has already been discussed.