The United Nations maintains peace by resolving disputes between nations. Its main objective is to prevent wars. It also engages in various humanitarian programs to encourage economic and social welfare around the world.
The United States considers the national security and good relationship with other nations as some of the most important elements of the nation, that's why a lot of its efforts, activities, and budget is meant to provide protection of its citizens to prevent a war from happening and to engage in humanitarian programs abroad. It does that by a variety of means, such as diplomacy, good trade relations, support of international bodies, by funding humanitarian organizations, military might, economic power, and power projection.