The First Seminole War (1817-1818) significantly influenced U.S. acquisition of Florida primarily through the last option: Spain realized it had little control over its land and agreed to sell it.
As tensions increased in the region due to conflicts involving the Seminole tribe, whose territory extended into Florida, American forces led by General Andrew Jackson conducted military campaigns against both the Seminole and Spanish settlements. The effectiveness of these military actions highlighted Spain's inability to maintain order and control over Florida, revealing its weak position in the region.
Faced with the reality of American military power and the challenge of governing an increasingly troubled territory, Spain was prompted to negotiate. This ultimately led to the Adams-Onís Treaty of 1819, where Spain ceded Florida to the United States in 1821. Thus, the First Seminole War played a crucial role in demonstrating that Spain could no longer effectively manage Florida, paving the way for its acquisition by the U.S.