History essay question about the use of treaty…?

History essay question about the use of treaty…?
Why did the US use the mechanism of the negotiated treaty with indian tribes to acquire legal title to a continent, land cession-by-land cession?
I have to write this essay for my final, thanks!

The US, president Jackson(Trail of tears) used treaties in order to expand westward.
The 1783 Treaty of Paris with Great Britain defined the original borders of the United States. Due to ambiguities in the treaty, the ownership of Machias Seal Island and North Rock remain disputed between the U.S. and Canada; other original territorial ambiguities (including the Northeastern Boundary Dispute and the disputed Indian Stream territory) were resolved by the Webster-Ashburton Treaty in 1842.
The Louisiana Purchase, completed 1803, was negotiated by Robert Livingston during the presidency of Thomas Jefferson; the territory was acquired from France for $11,250,000. A small portion of this land was ceded to the United Kingdom in 1818 in exchange for the Red River Basin. More of this land was ceded to Spain in 1819 with the Florida Purchase, but was later reacquired through Texas annexation and Mexican Cession.
West Florida was declared by President James Madison to be a U.S. possession in 1810.
Tristan da Cunha was the first, albeit short-lived U.S. overseas possession. This remote South Atlantic island was first claimed in 1810 by Jonathan Lambert from Salem, Massachusetts, who died in a boating accident in 1812. During the War of 1812, the U.S. used it as a naval and piracy base against British shipping. The island was abandoned after the war and annexed within months by the British, along with Ascension Island, in order to prevent the French from establishing bases from which to rescue Napoleon Bonaparte from Saint Helena.
Red River Basin, acquired in 1818 by treaty from the United Kingdom, namely the Anglo-American Convention of 1818.
The Adams-Onís Treaty of 1819 with Spain resulted in Spain’s cession of East Florida and the Sabine Free State and Spain’s surrender of any claims to the Oregon Country. Article III of the treaty, when properly surveyed, resulted in the acquisition of a small part of central Colorado.[1]
Webster-Ashburton Treaty of 1842 which finalized the border between United States and Canada (a British colony at the time).
Texas Annexation of 1845: In 1836 the Republic of Texas voted to be annexed by the United States. Despite the fact that Mexican leader Antonio López de Santa Anna warned that this would be “equivalent to a declaration of war against the Mexican Republic,” President John Tyler signed a treaty of annexation with Texas in April 1844. After James Polk, a strong supporter of territorial expansion, won the presidency, but before he took office, Congress approved the annexation of Texas on February 28, 1845. On December 29, 1845, Texas became the 28th state.
Oregon Country, the area of North America west of the Rockies to the Pacific, was jointly controlled by the U.S. and the United Kingdom following the Anglo-American Convention of 1818 until 1846 when the Oregon Treaty divided the territory at the 49th parallel (see Oregon boundary dispute). The San Juan Islands were claimed and jointly occupied by the U.S. and the U.K. from 1846-1872 due to ambiguities in the treaty (see Northwestern Boundary Dispute). Arbitration led to the sole US possession of the San Juan Islands since 1872.
Mexican Cession lands were a product of the Mexican-American War and the subsequent Treaty of Guadalupe Hidalgo, signed February 2, 1848. In this treaty, Mexico gave the U.S. parts of what is Texas, Colorado, Arizona, New Mexico and Wyoming, and the whole of California, Nevada and Utah and recognized the Rio Grande as Texas’ Southern border. The United States paid Mexico $15 million. In addition, the United States agreed to pay claims made by American citizens against Mexico, which amounted to more than $3 million.
Gadsden Purchase of 1853, United States purchased a strip of land along the U.S.-Mexico border for $10 million, now in New Mexico and Arizona. This territory was later used for the southern transcontinental railroad.
The Guano Islands Act of 1856 provided for U.S. claims to unoccupied islands. Baker Island, Howland Island, and Navassa Island were annexed in under its provisions in 1857. Today ownership of Navassa is disputed between the U.S. and Haiti. Johnston Atoll was claimed by the U.S. and Hawaii in 1858; the U.S. claim became undisputed in 1898 after the annexation of Hawaii. Midway Atoll was discovered and claimed in 1859 and formally annexed 1867. Kingman Reef was annexed in 1922.
Alaska Purchase from the Russian Empire for $7,200,000 in 1867.
National Atlas map (circa 2005) depicting territorial acquisitions and dates of statehood.
Chamizal from Mexico from 1852-1873 due to course change of the Rio Grande River. The territory was mostly retroceded to Mexico by treaty in 1963.
Hawaii, annexed 1898 upon the request of a government made up primarily of American and European businessmen who had overthrown the Kingdom of Hawaii. With Hawaii came the Palmyra Atoll which had been annexed by the U.S. in 1859 but later abandoned, then later claimed by Hawaii.
Puerto Rico, Gu