It would be the arrival of the "United States" in 1917 that was a turning point in the war (World War I) because it stopped the stalemate on the Western Front, since the US provided essential troops and war supplies.
The correct answer is the Adams-Onis Treaty.
This 1819 land agreement between the United States and Spain. This land treaty helped to establish a clear between Spanish and US territories. In this treaty, the US gained the rest of modern day Florida while the Spanish border would now definitively start at the Sabine River.
A notable difference between the spanish-american war and all of the earlier wars that the U.S. has fought was the location. It's because the United States planned to launch an attack to the Spaniards in the Philippines in 1898 - a place that's completely separate from the Americas during that time. The earlier wars happened in neighboring locations of the country, and within the country. The Spanish-American War was made possible due to the country's intervention in the Cuban War for Independence.
