There are many factors which contributed to the United States' imperialism in the late nineteenth and early twentieth centuries.  The Monroe Doctrine, originally meant to keep other nations out of American affairs became justification of American involvement outside of its borders.  The concept of manifest destiny originally meant to cover the continental United States was expanded to include areas in the Pacific Ocean and in Latin American.   The Industrial Revolution also played a part in imperialism because factories allowed the United States to mass produce goods and imperialism provided markets for the goods and sources of raw materials.  Americans also modeled the European imperialism and felt that they had to gain lands in other regions to remain competitive. 

This webquest will help you learn more about American imperialism during this time in history.