The origin of the Spanish-American war started with the struggle for Cuba to gain independence from Spain. The next 10 years were filled with "guerrilla warfare", and this was followed by a revolt by the Cubans in 1868. During the year 1895 is when the movement for independence continued, and because the Cuban's were struggling so much, they won the support of the United States.
On February 15, 1898, the US demands a intervention, but Spain denied this. There also was an explosion that destroyed the U.S.S. Maine, killing 270 people. After this, President McKinley asked Congress permission to declare war on the Spanish. The purpose of declaring war was to aid/help the Cuban patriots acquire their "liberty and freedom" that they were fighting for.
The Teller Amendment was then created and it stated that the United States were to have NO intention of annexing or dominating the island. The Secretary of State, John Hay, called the Spanish-American War the "Splendid Little War". It was called this because it only took four months, and there were less than 400 American combat deaths.
One of the most decisive battles occurred in the Philippine Island and not Cuba. This is where the American Navy defeated a Spanish fleet. There was another defeat off of Santiago, Cuba in July. Then they started to land more American troops on Cuba and Puerto Rico.
The most highly publicized battle was the charge up San Juan Hill by Theodore Roosevelt's "Rough Riders". Roosevelt believed that a war would reinvigorate the nation's unity. Because he wanted to raise a voluntary cavalry unit, he resigned his post as assistant secretary of the Navy. He rushed this unit into Cuba to help them fight, and after this he became a national hero. He later became the governor of New York, then the Vice President to McKinley.
The war escalated very quickly from aiding and helping the Cubans to an imperial risky journey. The Americans felt the need to train the Filipino people on how to self - govern themselves, so they don't get overcome by another nation.
In a treaty between Spain and the United States, that ended the war, the United States got to acquire the Philippines, Puerto Rico, and Guam. McKinley forced Cuba's new government to approve the Platt Amendment, which was to allow the United States military to intervene with their affairs whenever they see fit. They had to do this before they could have total independence.
The new land that the United States had were more about trade than anything else. Specifically, Puerto Rico and Cuba were gateways to Latin American. The Philippines, Guam, and Hawaii were trading gateways to Japan and China.
After the Spanish-American War, John Hay who was Secretary of State, announced the Open Door Policy. This was for the European granting the United States access to American exports for commercial sphere of influence, and this would mean the that United States would have some control on what goes on outside the borders that had to do with them. So basically, the movement of goods and money. This was sort of a problem because even though they demanded this, especially with China, the Chinese people were banned from immigrating to America.