The Adams-Onís Treaty

In 1819 the United States and Spain settled the long, tense question of where one ended and the other began. The Adams-Onís Treaty — negotiated by Secretary of State John Quincy Adams and the Spanish minister Luis de Onís — transferred Florida to the United States and drew a boundary line all the way to the Pacific.

Florida had been a running sore: a Spanish possession that the United States coveted and that Spain could no longer effectively control. Andrew Jackson's unauthorized 1818 raid into Florida made Spain's weakness plain, and rather than fight a war it could not win, Spain chose to sell.

In exchange for Florida, the United States assumed $5 million in claims its citizens held against Spain and gave up its tenuous claims to Texas — a concession that would help set up the conflicts of the 1830s and 1840s. The treaty also fixed a transcontinental boundary, the first time the United States had a defined claim reaching the Pacific.

For John Quincy Adams, it was a diplomatic triumph and a milestone in continental expansion. It cleared Spain out of the southeast, defined the nation's southern edge, and pointed American ambition toward the far western coast it would spend the next decades reaching.