Also called the Transcontinental Treaty of 1819, the Adams-Onis Treaty was one of the critical events that defined the U.S.-Mexico border. The border between the then-Spanish lands and American territory was a source of heated international debate. In Europe, Spain was in the midst of serious internal problems and its colonies out west were on the brink of revolution.
Facing the grim fact that he must negotiate with the United States or possibly lose Florida without any compensation, Spanish foreign minister Onis signed a treaty with Secretary of State John Quincy Adams. Similar to the Louisiana Purchase statutes, the United States agreed to pay its citizens’ claims against Spain up to $5 Million. The treaty drew a definite border between Spanish land and the Louisiana Territory.
In the provisions, the United States ceded to Spain its claims to Texas west of the Sabine River. Spain retained possession not only of Texas, but also California and the vast region of New Mexico. At the time, these two territories included all of present-day California and New Mexico along with modern Nevada, Utah, Arizona and sections of Wyoming and Colorado.
The treaty -- which was not ratified by the United States and the new republic of Mexico until 1831 -- also mandated that Spain relinquish its claims to the country of Oregon north of the 42 degrees parallel (the northern border of California). Later, in 1824, Russia would also abandon its claim to Oregon south of 54’40,’ (the southern border of Alaska.)
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