intervened in support of Panama's rebellion against Colombia in order to ensure the building of the canal. Here again, this was motivated by strategic calculation, for the Panama Canal promised to vastly shorten the distance it would take for ships to travel from the East Coast to the West. This had powerful implications, from both a military and commercial perspective.

The United States had reasons for getting involved in imperialism. We knew that to be a world power we needed to have land beyond our borders that we could control. All of the world power countries had colonies beyond their borders. If we wanted to be viewed the same way as the other world powers, we needed to get colonies. The opportunities to gain colonies were dwindling as other countries had already taken most land available for colonization.

Obtaining colonies would provide economic and political advantages for us. If we had colonies, we could get raw materials cheaper from the colonies than we could if we bought them from other countries. We also would have a place where we could sell our products made in our industries. By having colonies, our navy would have places around the world where our ships could refuel and resupply. These colonies could be used as a base for attack in case of war. By having these colonies, it would be easier for our navy to protect our trade. We knew that world power countries must be able to protect their trade.

There were people who believed it was our duty to spread our way of living around the world. They now believed that the concept of Manifest Destiny should be spread beyond our borders. These people believed it was our obligation to spread our superior way of life and culture to other people throughout the world.

When we saw an opening to gain colonies by going to war with Spain, we seized that opportunity. We believed over-exaggerated stories about Spanish mistreatment of the Cubans. We jumped to conclusions that Spain was responsible for the explosion of the U.S.S. Maine. As a result of the Spanish-American War, we gained colonies, became an imperial power, and assumed our place as a world power.

There are three main reasons why the US engaged in imperialism.

1. Sources for raw materials, new markets for finished goods.  The colonial economic theory of mercantilism explains that a "mother country" sets up a colony that can grow/provide raw materials or goods.  Those goods can only be traded back to the mother country.  This made mother countries extremely profitable.  When the US gained control of new territories, they could not only have access to new goods, but also have new markets to sell its goods.
2. Extending power/military reach.  By setting up colonies around the globe, the US would have potential military bases that could be used in times or war or conflict.  Even today, the US has military bases in sovereign nations around the world.  This way, if conflict arises and the US intends of sending in troops, they could pull from the closest available base.  This was especially useful during WWII when the US used naval bases in the Pacific (Hawaii, Midway, Guam) to better reach Japan.
3. Cultural assimiliation.  The US (and European powers) have a long history of wanting to "westernize" cultures.  Rudyard Kipling wrote it best in "The White Man's Burden", characterizing cultures in Africa as uncivilized and barbaric, needing of Western humanitarianism and culture.   

There are generally said to be three reasons for this.

First, the US got involved for economic reasons.  Americans felt that taking places like Hawaii and the Philippines would give them new sources of raw materials.  It would also give them new markets in which to sell their goods.

Second, Americans wanted to spread their way of life around the globe.  They felt that they were superior to other countries and that it would be good to bring things like democracy and (their form of) Christianity to other countries.

Finally, Americans wanted more military power.  Alfred Thayer Mahan had argued that sea power was vital to world power.  By taking an empire, the US enhanced its sea power by giving it bases around the world.

The late-nineteenth century United States became an imperial power for a multitude of reasons.

The nineteenth-century doctrine of Manifest Destiny was a contributing factor to American imperialism. Manifest Destiny was the idea that American expansionism was both right and inevitable. By the late nineteenth century, the United States had reached its territorial limits in North America. Therefore, further expansion overseas was a natural continuation of Manifest Destiny.

In 1890, Alfred Thayer Mahan (1840–1914) published The Influence of Sea Power upon History, 1660–1783. Mahan, an American naval officer and instructor, advocated both a strong navy and colonial expansionism. Great nations had strong navies, so an aspiring US needed one too. Mahan's ideas were influential both abroad and in the US America's Secretary of the Navy, Theodore Roosevelt, was influenced by Mahan.

In the late-nineteenth century, Asia and Africa were carved up by European powers. They sent missionaries to convert heathen peoples, but these nations were more interested in political and economic aggrandizement. Influential members of the government of the United States—such as Senators Albert Beveridge and Henry Cabot Lodge—decided to emulate the Europeans.

Social Darwinism played a role in American imperialism. Proponents of Social Darwinism believed the Anglo-Saxon race was superior to others. Therefore, it had a duty to civilize inferior peoples.

Late-nineteenth century American imperialism focused on the Pacific. Samoa and Hawaii were taken over. In 1898, America defeated Spain and received the Philippines, Guam, and Puerto Rico.

The United States wanted to become an imperial power in the late 1800s. There were several reasons for this. One of our goals was to become a world power. In order to do this, we needed to have colonies beyond our borders. Since most of the available land was already colonized, we were going to have gain colonies in a war. By winning a war and gaining colonies, we could become a world power.

We also knew that by having colonies, it would give our navy a place to stop and resupply and refuel their ships. We could also use the colonies as a base of our military operations if needed. Having colonies would make it easier for our military to take actions that could maintain our world power status once that status was achieved.

We also wanted resources and places to sell our products. If we had colonies, we could get needed resources cheaper from the colonies than by buying them from other countries. Also, we would be able to sell our products to the colonies. This would give our businesses a guaranteed market for their products.

Finally, we believed our way of life was superior to the way of life in other countries. We believed it was our duty to spread our superior way of life to these countries. We believed we needed to show them how to govern themselves and how to develop an economy. There were many reasons why we wanted to become an imperial power in the late 1800s.

There are several reasons that the United States got involved with imperial colonies throughout the world.

1. New markets for goods, new lands for natural resources.  As in the case of all imperial colonies (including us when we were a British colony!), having new land has broad economic opportunities.  Not only can the new land provide different resources and raw materials with which the mother country can make luxury manufactured goods, but a new colony is also a new market that can be made to trade only with the mother country. 
2. Military and naval bases, extending the military's reach throughout the world.  The US received Guam as a ceded territory from the Spanish-American War.  Later, during WWII, this became a location that the US could gather troops and ships during its island hopping campaign in the Pacific Ocean.  Even today, the US has dozens of bases stationed throughout completely sovereign nations in the world as a way of maintaining a military force close by to any potention hot spots.
3. The need to "civilize" other cultures.  The best example of this is European "humanitarian" interventions in Africa, as best described by Rudyard Kipling's poem "The White Man's Burden".  However, we can also look at US intervention in the Philippines.  While we initially supported the Philippine's independence movement against Spain, we held on to the land once we defeated the Spanish.  Several documents were released that the Filipinos were not civilized as the Americans were, so we were not willing to give the Filipinos their freedom.  I personally enjoy this quote from Andrew Carnegie, an anti-imperialist in the last 19th and early twentieth century:

"You seem to have about finished your work of civilizing the Filipinos.  About 8,000 of them have been civilized and sent to Heaven.  I hope you like it."

Three reasons are typically given for American imperialism in the late 1800s and early 1900s.

Economic factors.  The US wanted to take an empire to give it new markets in which to sell goods and from which to buy raw materials.

Military factors.  This was the time of Alfred Thayer Mahan’s ideas about the dominant role of naval power.  The US felt that taking an empire would give it more naval bases from which to project power.  This would allow the  US to become a more important power.

Cultural factors.  Many Americans believed in the idea of the “white man’s burden.”  They felt that they needed to go out and help to civilize the rest of the world.