In a sense, the United States has been "expansionistic" from its very beginning. The 13 English colonies, clinging to the eastern seaboard, were determined to push westward despite all natural and political obstacles. Once established as a nation, the United States went about acquiring even more land, including Florida, the Louisiana Purchase, and the Mexican Cession. The expansion associated with the late nineteenth century was just one chapter in a long book.
One can begin writting a paper anout expansion of the U.

S. beginning with the founding of the colonies. Colonial expansion involved many resons- land hunger, flight from religious persecution, etc.-
The next main expansion came with the Louisiana Purchase in 1803.

Then the Annexation of Florida, Oregon, and Texas (1845). The Gasdesn Purchase 1853. Manifest destiny (1840's-1850's). And the Annexation of Hawaii.A few more points of interest one can use in writting an essay concerning U.

S. expansion are:
Imperialism, Darwanism, Open door policy, Defense, How the Spanish American had broken out and expansionist arguments had renewed force, Roosevelt Corollary, the Monroe Doctrine, contiguous states vs. non contiguous states, Diety, the Platt Amendement, Filipinos, and "new imperialism"
These are just a few of the ideas that could be included in an essay concerning expansionism.
Bibliography:
Doing the DBQ