Final answer:
The United States acquired Florida from Spain through the Adams-Onís Treaty but had to conduct military actions against the native Seminole tribes. Other territories gained after the Spanish-American War include Cuba, Puerto Rico, Guam, and the Philippines.
Step-by-step explanation:
The territory that the United States acquired from Spain but had to engage in conflicts with the natives to secure was Florida. As per the Adams-Onís Treaty of 1819, the United States gained control of Florida in exchange for $5 million.
However, it was not a peaceful transition as conflicts with the natives ensued, notably with Seminole tribes during the Florida Wars.
Following the Spanish-American War, the United States also gained possession of Cuba, Puerto Rico, Guam, the Philippines, and various other islands.
Although Puerto Rico and Guam remain U.S. territories, the Philippines and Cuba eventually became independent nations.
The aftermath of the war required the U.S. to deal with further challenges, including conflicts with the native populations in newly acquired territories.
The circumstances surrounding the acquisition of these territories illustrate the complexities of American expansionism and colonialism during the 19th and early 20th centuries, including the interplay between diplomatic treaties and military actions.