Originally Answered: The period 1881-1919 marked a fundamental change in American foreign policy. Explain the changes that occurred
By 1881, most of the North American continent had been explored and brought under relative control by Europeans, and the government could then begin to address issues beyond our shores.
U.S. Foreign policy from this period until now is imperialism with a small "i", that is a kind of imperialism to which we will not admit officially. Prior to this, there were some instances of this role in the western hemisphere. Our issuance of the Monroe Doctrine, in 1821, placed the US in the role of a kind of elder brother defending his younger siblings on the schoolyard against the bullies from across the Atlantic. Our actions toward the independent republics formed in the years before the Civil War were as a kind of tutor in the school of democracy. Henry Clay, as Secretary of State under Adams, took great pride in his role in the movement of "Pan-Americanism".
When the War ended, the US could look beyond thus role into the realm of helping others to liberate themselves from their European overlords. Cuba was our first target, with Spain as an aging power, we chose to assist Cuban rebels in their effort to free themselves. In addition, since we were taking on Spain in the first place, military needs made it reasonable to take the Philippines as well. There was an independence movement in Cuba, but it was in an adolescent stage compared to the mature stage found in the Philippines. The military actions in Cuba were brief, since the Spanish were better armed, but not as numerous as the norte americanos. In the Philippines, Admiral Dewey had the assistance of Filipino rebels assaulting from the land, while the US Naval flotilla assaulted from Manila Bay. This fight against the Spanish was fairly easily won as well. What happened, was that we had reportedly agreed to support the independence movement in the Philippines, and we renegged on the agreement. From that point on, we were fighting the Filipinos, off and on, until both sides were distracted by the Japanese in 1941.
The taking of the Philippines gives us a naval base in a position between east Asia and Oceania. From San Francisco to Manila is nearly 7,000 miles. We need something closer if we are to have more control over the Pacific (oops, sorry we don't want to control the Pacific, we just want to protect our interests) we need something closer. So, we bought Hawaii. Honolulu is only 2,400 miles from San Francisco. In addition, we gain control over the islands of the Pacific we have today, places like Midway and Guam.
Now McKinley is not as much a player here as others are, illness plays a role in his administration. His wife had nearly died just a few months before he himself was assassinated.
Roosevelt is a peculiar critter. He starts out in life as a sickly boy, son of someone who paid a substitute in the Civil War, who built himself up through a vigorous life, organized a volunteer company to take to Spain, lavishly praised the Colored cavalry who joined in the attack on San Juan Hill and el Caney, yet turned his back on African Americans when he was in the White House, built the US Navy into a two ocean navy by sending our "great white fleet" halfway round the world and then waited for Congress to come up with the money to bring them back, he was a trustbuster, I think from a sincere belief in fair play, but he was also an agitator, founding the Plattsburg movement to train soldiers for the war in Europe when the policy was to "remain neutral in our hearts and minds".
I've always seen Wilson as a kind of misplaced scholar. He had strong beliefs, but actuating those beliefs are not always easy. His response to the incursions of Villa into the United States failed, of course, though it could have expanded the world conflict into Mexico, had Herrera had any real support base to respond. He was forced into the war by the German efforts to bring Mexico into the war. His Fourteen Points was a brilliant effort, which could have brought Peace in our time, had there been any real effort to acknowledge the real world of politics. Had he invited Lodge along to the conference, he might have helped win US. ratification of the treaty. The "ivory tower" viewpoint had the same results in Europe. The US had sent troops into action for only about a year, while France and Britain were being bled white for four years.
He took the role of peacemaker, trying to protect the world from the bullies in the schoolyard, but he neglected to notice the bloody noses of the bullies on his own team, and their desire to "get a little of their own back"
On US policy in the Philippines, I strongly recommend the essays of Mark Twain. He became rather unpopular in this time because he stopped writing funny books, and started attacking government policy on this issue.