Florida
Florida had become a burden to Spain, which could not afford to send settlers or garrisons. Madrid therefore decided to cede the territory to the United States through the Adams–Onís Treaty, which took effect in 1821. President James Monroe was authorized on March 3, 1821 to take possession of East Florida and West Florida for the United States and provide for initial governance. Andrew Jackson, on behalf of the U.S. federal government, served as a military commissioner with the powers of governor of the newly acquired territory for a brief period. On March 30, 1822, the U.S. Congress merged East Florida and part of West Florida into the Florida Territory.
By the early 1800s, Indian removal was a significant issue throughout the southeastern U.S. and also in Florida. In 1830, the U.S. Congress passed the Indian Removal Act and as settlement increased, pressure grew on the U.S. government to remove the Indians from Florida. Seminoles offered sanctuary to blacks, and these became known as the Black Seminoles, and clashes between whites and Indians grew with the influx of new settlers. In 1832, the Treaty of Payne's Landing promised to the Seminoles lands west of the Mississippi River if they agreed to leave Florida. Many Seminole left at this time.
St. Augustine, Florida
During the Second Spanish period (1784-1821) of Florida, Spain was dealing with invasions of the Iberian peninsula by Napoleon's armies in the Peninsular War, and struggled to maintain a tenuous hold on its territories in the western hemisphere as revolution swept South America. The royal administration of Florida was neglected, as the province had long been regarded as an unprofitable backwater by the Crown. The United States, however, considered Florida vital to its political and military interests as it expanded its territory in North America, and maneuvered by sometimes clandestine means to acquire it. The Adams–Onís Treaty, negotiated in 1819 and ratified in 1821, ceded Florida and St. Augustine, still its capital at the time, to the United States.