There were a number of arguments made in favor of US expansion and imperialism in the late 19th and early 20th centuries. Let us look at the most important of these.
Imperialism was good because it would help the US economically. At this time, the US was getting stronger economically and it felt that it needed an empire to help continue that growth. It believed that an empire would be a good source of raw materials and a good market to which it could sell manufactured goods. Thus, imperialism would help the US economy grow.
Imperialism was good because it would make the US stronger militarily. This idea is closely connected with the military thinker Alfred Thayer Mahan. According to this idea, world power rested on sea power. If the US took an empire in the Pacific, it would have many naval bases from which it could project military power. This was important because the US needed to keep up with European countries whose empires were growing. Thus, imperialism would help the US become a world-class military power.
Imperialism was also perceived as good because it would help the people whose countries were being taken. This idea is often termed the “white man’s burden.” The idea here was that Americans (and other whites) were racially superior to the non-white people of Asia and the Pacific. By taking an empire, the US would be helping these people. It would bring them to a much higher level of civilization and prosperity than they could achieve on their own. In this way, imperialism was considered good because it would allow the conquered people to be connected to a superior civilization.
In short, Americans argued that imperialism would be a good thing because it would help the US militarily and economically and because it would help to improve the lives of the colonized peoples.