One reason Americans did not go back east was that they were too busy going farther west. The end of Europe's wars freed Americans to concentrate on what they and their ancestors had been doing since the start of the seventeenth century: claiming new territory and dispossessing the aboriginal inhabitants. Independent America inherited from imperial England the general idea that Indians had no rights worth respecting; Indian lands might be claimed and parceled out, subject only to the ability of the claimants to drive the Indians off. This attitude was not quite universal; a few English and Americans spoke up for the Indians. But between the more common view of Indians as beneath respect, and the ravages of introduced diseases upon the aboriginal populations, Americans expanded west almost as though the Indians were not there.
The Treaty of Paris of 1783 confirmed American title to the lands lying east of the Mississippi. (Confirmed title against European claimants, that is. No Indians sat at the Paris negotiations, and none were asked to approve the treaty.) The Louisiana Purchase of 1803 doubled the American national domain again, pushing the western boundary to the crest of the Rocky Mountains. Diplomatic and extra-diplomatic machinations brought Florida into the American fold.
Nearly all Americans approved this heady expansion. Federalists poked fun at Jefferson, who had to abandon his strict constructionism lest Napoleon change his mind about Louisiana; and General Andrew Jackson incurred criticism for hanging one British trader and shooting another in Florida. But on the whole, more land was conceived to be in the obvious national interest.
There was something else behind the proexpansionist feeling. Since Puritan times, many Americans believed their country was unique: the site of the terrestrial working-out of the will of God (either the Christian God of the Puritans or the natural God of Jefferson and Madison). The expansion of America, in this view, signified the extension of providential designs, with benefits not merely for America but for humanity at large. Other nations have defined their national interests to include people beyond their borders, but none have done so more vigorously or consistently than the United States.
The identification of the American national interest with the interest of humanity reached an early apex during the 1840s, in the era of "manifest destiny." The annexation of Texas, the conquest of California and New Mexico, and the acquisition of Oregon seemed to most Americans a convenient collaboration between God and the United States. To be sure, the war with Mexico occasioned complaint among northern Whigs, but their complaint had less to do with the extension of American authority than with a fear that the territory acquired in the war would strengthen the institution of southern slavery.
In fact, slavery never sank roots in the trans-Texas southwest. Yet difficulties in organizing governments for the Mexican cession did reopen the slavery debate and led, via "bleeding Kansas" and the Dred Scott decision, to the Civil War. The bitter sectional ordeal took the smugness out of the manifest destinarians and the wind out of the sails of territorial expansion. The generation that came of age with the Compromise of 1850 and remained in power until the Compromise of 1877 had trouble enough holding together the territory America already owned; it had little energy or inclination to acquire more. Secretary of State William Seward got Congress to purchase Alaska in 1867, but only because Russia's sale price made it a bargain the legislators could not refuse (especially when the price included generous kickbacks to the legislators themselves). Ulysses Grant tried to annex Santo Domingo, but the Senate said no.