The most important outcome of the adams-onis treaty of 1821 was that

florida became a territory of the united states.
spain no longer had a us presence.
spain gave up rights to alabama and georgia.
florida became the twenty-seventh state to enter the union.

Spanish minister Do Luis de Onis and U.S. Secretary of State John Quincy Adams sign the Florida Purchase Treaty, in which Spain agrees to cede the remainder of its old province of Florida to the United States.

Spanish colonization of the Florida peninsula began at St. Augustine in 1565. The Spanish colonists enjoyed a brief period of relative stability before Florida came under attack from resentful Native Americans and ambitious English colonists to the north in the 17th century. Spain’s last-minute entry into the French and Indian War on the side of France cost it Florida, which the British acquired through the first Treaty of Paris in 1763. After 20 years of British rule, however, Florida was returned to Spain as part of the second Treaty of Paris, which ended the American Revolution in 1783.

Explanation:

Website: https://www.history.com/this-day-in-history/the-u-s-acquires-spanish-florida

If helpful mark brainest thx

a. because the u.s. was the leading with the most technology, economy, etc. also was teamed with other allies witch boosted the economy and materials to improve technology and use it.