Under the terms of the Adams-Onís Treaty, ratified in 1821 between Spain and the United States, Florida was ceded to the United States in exchange for U.S. forfeiture of a $5 million debt owed by Spain. Florida became a U.S. Territory in 1821. By 1830, the de facto leader among the approximately 70 people living at the "New River Settlement" (present day Fort Lauderdale) was William Cooley. Cooley was appointed by Governor William Pope Duval as Justice of the Peace for the region.
In 1835, white settlers killed a Seminole chief named Alibama and burned his hut in a dispute. As Justice of Peace, Cooley jailed the settlers, but they were released after a hearing at the Monroe County Court in Key West; the justification was insufficient evidence. The Seminoles blamed Cooley, saying he withheld evidence. The growing uneasiness between the Seminoles and the whites led to the Seminole migration to the Lake Okeechobee area. On 28 December 1835, a Seminole ambush known as the Dade Massacre started the Second Seminole War.
On 3 January 1836, Cooley led a large shipwrecking expedition from the settlement to free the Gil Blas, a ship that had beached the previous September; the scale of the operation required most of the settlement's able men. The following day, a group of 15 to 20 Seminoles invaded the Cooley house, killed Cooley's wife and children, scalped the children's tutor, and burned the house to the ground. Although the Indians did not attack any other families, the massacre triggered the departure of the white settlers from the area. During the second Seminole War, Major William Lauderdale led his Tennessee Volunteers into the area. In 1838, Lauderdale erected a fort on the New River at the site of the modern city of Fort Lauderdale (where SW 9th Avenue meets SW 4th Court). Lauderdale left after one month, but his name remained. The Seminoles destroyed the fort a few months later. Two more forts were built sequentially, each closer to the ocean. After the end of the Second Seminole War in 1842, the fort was abandoned, and the area remained largely empty, as the remaining Seminoles withdrew to Pine Island, and only a handful of settlers were known to live in all of what eventually became Broward County. While the area was technically a part of the Confederacy during the US Civil War, the only known white settlers in the area during the war was pro-unionist Isaiah Hall and his family, who had been run out of Miami by pro-confederacy sympathizers in 1863, and settled on the New River.
As there was no overland route into or out of the area, no significant settlement was undertaken until the 1890s. In 1892, however, the first road through the county was built, when a road was constructed from Lemon City, a settlement near the town of Miami, to Lantana, on the southern shore of Lake Worth, in Palm Beach County. A ferry crossing was established across the New River.