The United States has often seen itself as the standard in human rights and democracy. Going back to Manifest Destiny, the United States saw itself as destined to spread its ideals throughout North America. In return for spreading its ideals, the United States hoped to gain commercial benefits such as fertile soil, minerals, and other natural resources.
This attitude continued after the Frederick Jackson Turner called the frontier "closed" in 1890 as the United States sought to expand its might abroad. The United States was heavily influenced by Rudyard Kipling's "The White Man's Burden" to spread "civilization" throughout the developing world. The United States was also influenced by Mahan's paper on the importance of a strong navy to secure new markets and resources.
The United States went into the Spanish-American War as both a humanitarian effort to protect Cubans from Spanish atrocities and to protect American business interests. In both WWI and WWII, Americans stepped in to fight German expansion in what they viewed as a threat to democracy. In WWII, Americans also fought against Japanese oppression. During the Cold War, Americans waged war against the Soviet Union, which they viewed as the antithesis of democracy and the primary human rights violator in the world.
In the current War on Terror, the United States has gone to war not only to stamp out extremist groups but to bring democratic principles to Iraq and Afghanistan. While the United States has often not lived up to its own democratic ideals, American exceptionalism has a long tradition in American foreign policy.