The correct answer to this open question is the following.
Although you did not specify the time in history or any specific context, we are going to assume that you are referring to the US imperialistic role through history.
Being that the case, we can comment on the following.
The foreign policy actions of the United States reflect selfish, imperialist ambitions since the times of President James Polk and the Manifested Destiny. He expanded the US territory acquiring the Mexican northern states after the Mexican-American War.
Since those years, passing through the times of President Monroe and the Monroe Doctrine and President Theodore Roosevelt and his Roosevelt corollary, the United States has always had interventionist purposes as part of its foreign agenda.
Nobody has granted the US the right or role to be the "big brother" who was looking after the interests of the western hemisphere. That is why the US has had many problems throughout history in the Middle East, Latin America, Europe, and other regions.
This region was also politically unstable as there were different ethnicities and there was also rising nationalism in the region. Nationalism brought about tensions. There was also rising Serbian nationalist groups. Combined with the brinkmanship of the European Powers the area became a fuse to the powder keg that Europe had become. The Balkans has strong Cultural, Religious, and language ties to the surrounding Empires but they were in small pockets making nation building difficult.
Sometimes it's for religions . <span />
The transcontinental railroad transformed the west because it made the idea of manifest destiny so much easier to achieve. The railroads also gave access to many trade spots, which in result connected them all, making the economy of America BOOM.
