Explain clearly why the United States started to look to expand overseas during the late 1800s.

There are many reasons why the United States began to look to expand overseas, a process known as "imperialism," in the late nineteenth century. Let us look at a few. 

First, to look at ideological factors, the United States looked overseas because the nation's conquest of the lands west to the Pacific Ocean was complete by midcentury, and especially after victory over the Plains Indians in the 1870s. Many who saw America's destiny in terms of expansion began to look beyond North America. The late nineteenth century was also a period when many people were influenced by Social Darwinism, and many viewed the world in racial terms. They believed that the United States had the right and the obligation to spread its influence around the world. Many argued that imperialism would actually be beneficial to the peoples under American control--allegedly superior American white Christians would bring their culture to allegedly primitive peoples around the world. This concept is often called the "white man's burden," and it was a powerful argument for imperialism.

The desire for expansion was also stoked by business interests. American business leaders sought secure markets for their manufactured goods, and the best way to gain them was, they thought, through the use of the US military (for example, in several Central American countries), or at least through aggressive diplomacy (for example, in China). Some territories, like Hawaii, were annexed largely due to the influence of business interests. Wealthy American planters on the island thought an American presence would benefit their business interests. Most of them were sugar and fruit planters, and their influence also underscores another reason for US imperialism--the acquisition of cheap raw materials and labor for goods to be sold in the United States and abroad. 

Finally, imperialism was viewed as a military and strategic necessity. The book The Influence of Naval Power Upon History, published by Alfred Thayer Mahan in 1890, convinced many in the US government that the nation would have to expand its naval power if it was to join the great nations of the world. To have a navy, the United States needed friendly ports and coaling stations around the world. For this reason, the nation expanded into the Pacific, acquiring small islands. The expansion of naval power made the expansion of economic and diplomatic power possible as well.