From a Q & A with Walter Nugent, author of Habits of Empire: A History of American Expansion:
Q: When did the United States first start to demonstrate its “habit” of empire-building? Have there been any moments in history when the country might have kicked the habit, but didn’t?Read the complete Q & A.
A: The seeds of the habit go back to the colonial period, when Anglo-Americans pushed back the Native occupants on all fronts. My book starts with the settlement in 1782 following the Revolutionary War, where the American negotiators, Franklin, John Adams, and Jay, pushed very hard for all the territory they could get. They succeeded in establishing our western boundary at the Mississippi, despite the fact that we had not conquered and had not occupied Transappalachia, the huge region between the eastern Mountains and the great River. Americans tried but never succeeded in obtaining Canada, either in the 1782 settlement (though Franklin pushed very hard) or in the Canadian-American war of 1812-1814. But they spread successfully westward and southward, reaching the Gulf of Mexico, the Rio Grande, and the Pacific by 1848. Each territorial acquisition between 1782 and 1848 reinforced the imperial habit. After that march across the continent, the habit was ingrained, and empire-building proceeded offshore from Alaska to the Philippines and around the Caribbean by the early twentieth century, and since 1945 around the world.
The Page 99 Test: Habits of Empire.