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The Adams-Onís Treaty of February 12, 1819, also known as the "Transcontinental Treaty," which settled border disputes between the United States and the Spanish Empire, proved vital to the nation's security. The catalyst for the negotiations between U.S. Secretary of State John Quincy Adams and the Spanish minister to the United States, Luís de Onís y Gonzalez, was border raids by Seminoles out of Spanish Florida. Once negotiations over Florida commenced, the issue of borders in the Great Plains and Pacific Northwest came to the fore.
Central to Spain's willingness to accept Adams's border demands in the Great Plains was the secretary of state's eventual willingness to give up claims on Spanish Texas. After lengthy negotiations, Adams and Onís established a Louisiana Purchase boundary line that followed the Sabine, Red, and Arkansas Rivers northwest to the forty-second parallel and then straight west to the Pacific. In accepting this line, Spain relinquished all claims to the Pacific Northwest, thereby improving the chances of the United States gaining control of the region.
The most significant element of the treaty was Adams's insistence that the boundary be extended to the Pacific. Adams's skillful negotiations opened the way for the United States to become a transcontinental power. In asserting for the United States a continental presence, Adams secured what he believed to be his most important diplomatic achievement, and one that marked a new era in U.S. history.