I need to know more about a period in the history of United States called "American Empire," which starts in 1898 and ends in 1914. What were its reasons and, mainly, its consequences for the world scenario? In my paper I also need to comment about at least 1 original document of this period that might be any kind of source as long as it fits this period of American Empire. So I need your help to suggest an original document that is relevant to this period and comment about it. Thank you!

During the period from 1898–1914, the United States began to expand its reach overseas; therefore, the era is known as that of the "American Empire," beginning with the Spanish-American War in 1898. The reasons for American imperialism were many and included a desire to compete with other nations, mainly in Europe, for world power and resources. It also included the desire to gain access to other markets and to overseas ports.
Teddy Roosevelt, who was Assistant Secretary of the Navy when the Spanish-American War broke out and later became President (in 1901), advocated the development of an American empire. In his speech "The Strenuous Life," given in 1899, he spoke of his support for imperialism:

For as the nations grow to have ever wider and wider interests, and are brought into closer and closer contact, if we are to hold our own in the struggle for naval and commercial supremacy, we must build up our power without our own borders. We must build the isthmian canal, and we must grasp the points of vantage which will enable us to have our say in deciding the destiny of the oceans of the East and the West.

In this speech, Roosevelt advocated the growth of imperialism to advance American economic and naval interests, and he supported the idea of building a canal to connect the Atlantic and Pacific coasts of the United States. This would later become a reality when he supported the building of the Panama Canal as president. The complete speech, which could be your primary source, is at the link below. The results of this period were increased American power abroad and American intervention in Latin America.
http://voicesofdemocracy.umd.edu/roosevelt-strenuous-life-1899-speech-text/