The annexation of Texas inevitably led to a movement on California, which could be obtained only by aggressive war upon Mexico, and for connection with which the possession of New Mexico was also thought necessary. Ever since 1820 explorers had been opening up the region between the Mississippi and the Pacific,[14] and it was known that there were several practicable roads to that distant coast.[15] The annexation of California almost led the United States into a serious territorial adventure; for apparently nothing but the hasty treaty negotiated by Trist in 1848 stopped a movement for the annexation of the whole of Mexico.[16] The Gadsden Purchase of 1853 conveniently rounded out the cession of New Mexico and closed this second era of territorial expansion.
The annexation of Texas was logical, and delayed only by the accidental connection with slavery; but the annexation of Oregon and California added to the Union very distant possessions, the settlement of which must have been slow but for the discovery of gold in California in 1848. At once a new set of territorial questions arose: the necessity of reaching California across the plains led to the organization of Nebraska and Kansas territories in 1854, which convulsed the parties of the time; the movement across the Isthmus to California brought up the question of an interoceanic canal in a new light; the commercial footing on the Pacific led to a pressure which broke the shell of Japanese exclusion in 1854. Above all, these annexations brought before the nation two questions of constitutional law, which proved both difficult and disturbing: the issue of slavery in the territories, which precipitated, if it did not cause, the Civil War, and the eventual status of territories which, from their situation or their population, were not likely to become states.
The third era of national expansion began in 1867 with the purchase of Alaska,[17] which was wholly a personal plan of Secretary Seward, in which the nation took very little interest; nor was the public aroused by Seward’s more important scheme for annexing the Danish West India Islands and a part of Santo Domingo; when the latter project was taken up in 1870 and pushed with unaccountable energy by President Grant,[18] popular sentiment showed itself plainly averse to annexing a country with a population wholly negro and little in accord with the American spirit. For twenty-five years thereafter there was the same indisposition to annex territory that brought problems with it; and then the movement for the annexation of Hawaii was headed off by President Cleveland in 1893.[19] The Spanish War of 1898 swept all these barriers away, and left the United States in possession of the Philippine Islands, a distant archipelago containing seven and a half millions of Catholic Malays; of the island of Porto Rico, in the West Indies; of the Hawaiian group; of a responsible protectorate over Cuba; and, four years later, of the Panama strip, which may include the future Constantinople of the western world.